Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/419

CETACEA.] The type of this genus is Z. cavirosfris of (Javier, founded upon an imperfect skull picked up in 1804 on the Mediterranean coast of France, and described and figured in the Osscmcns Fossilcs under the impression that it was that of an extinct species. Many other individuals have, however, been subsequently met with in various parts of the world, from the Shetland Islands to New Zealand, all referable to the same genus if not to the same species ; although, as is usual in such cases, they have mostly been described under different names. Teeth, apparently of allied forms, are abundantly found in the Suffolk and Antwerp Crags. Mesoplodon. 1 A much compressed and pointed tooth in each ramus of the mandible, variously situated, but generally at some distance behind the apex ; its point directed upwards, and often somewhat backwards, occasionally developed to a great size. Skull with the region around the nares as in Hyperoodon, except that the nasals are narrow and more sunk between the upper ends of the FIG. 45. Mesojilodon bident. Fivm Keinhurdt. premaxilla? ; like those offfyperoodon,ihey are concave in the middle line in front and above. No maxillary tuberosities. Anteorbital notch not very distinct. Rostrum long and narrow. Mesethmoid in adult age ossified in its entire length, and coalescing with the surrounding bones. Vertebra : C 7, D 10, L 10 or 11, U 19 or 20 ; total 46 to 48. Two or three anterior cervicals unibed, the rest usually free. Though varying in form, the mandibular teeth of the different members of this genus agree in their essential structure, having a small and pointed enamel-covered crown, composed of true dentine, which, instead of surmounting a root of the ordinary character, is raised upon a solid mass of osteodentine, the continuous growth of which greatly alters the form and general appearance of the organ as age advances, as seen most strikingly in the case of M. layardi, where the long, narrow, flat, strap-like teeth, curving inwards at their extremities, actually meet over the rostrum, and must greatly interfere with the movements of the jaw. In one species (M. grayi) a row of minute, conical, pointed teeth, like those of ordinary Dolphins, 17 to 19 in number, are present even in the adults, on each side of the middle part of the upper jaw, but embedded by their roots only in the gum, and not in bony alveoli. This fact, with the frequent presence of rudimentary teeth in other species of this and the last genus in both upper and lower jaws, suggests the idea that the Ziphioids are derived from ancestral forms having teeth of normal character in both jaws, but whose dentition FIG. 46. Skull of Mcsoplodon densirostris. has become greatly specialized. The existing species of this genus are widely distributed in both northern and southern hemispheres, but most frequent in the latter. The best established are M. lidcns, M. curopasus, M. densirostris, M. layardi, M. grayi, and M. hcctori ; but there is still much to be learned with regard to their distinctive characters and geographical distribution. They were abundant in the Later Miocene and Pliocene age, as attested by the frequency with which the most imperishable and easily recognized portion of their structure, the long, cylindrical rostrum of the skull, of more than ivory denseness, is found among the rolled and waterworn fragments of animal remains which compose the well-known "bone-bed" at the base of the lied Crags of Suffolk. Numerous generic distinctions have been founded upon slight modifications of the form of these rostra, such as Choneziphius, Placoziphius, Bdcmnoziphius, &c. , but these can only be considered as provisional until further knowledge is obtained of the structure ot the animals to which they belonged. 1 For the very complicated synonymy of this genus, see Trans. Zoo!. Soc., voL vi.i. p. 208. 397 Berardius. Two moderate-sized, compressed, pointed teeth on each side of the symphysis of the mandible, with their apices directed forwards, the anterior being the larger of the two and close to the apex. Upper ends of the premaxilhie nearly symmetrical, moderately elevated, very slightly expanded, and not curved forward over the nares. Nasals broad, massive, and rounded, of nearly equal size, forming the vertex of the skull, flattened in front, most prominent in the middle lino. Anteorbital notch distinct. Rostrum long and narrow. Mesethmoid only partially ossified. Small rugous eminences on the outer edge of the upper surface of the maxilla; at base of rostrum. Vertebrae: C 7, D 10, L 12, C 19 ; total 48. The three anterior cervicals ankylosed, the rest free and well developed. The only known species, B. arnouxi, attains the length of 30 feet, and has hitherto only been met with in the seas around New Zealand. Family SQUALODOKTIDJE. Numerous extinct forms, chiefly known by teeth nnd fragments of crania, may be provisionally placed here, until more of their osteological characters shall be brought to light. They differ from all existing Cetaceans in having the teeth distinctly differentiated into groups, as in the Archseoccti, the posterior molars being two-rooted. The cranium, has, however, none of the distinguishing characteristics of the Zcuglo- dons, but essentially resembles that of the Odontoccti, especially in the position of the anterior nares and form of the nasal bones. The best-known are associated in the genus Squalodon. Denti tion : i f , c -J-, simple teeth of the molar scries (premolars ?) |, two- rooted molars | = |-| ; total 60. The double-rooted molars differ from those of Zcuglodon in having the denticulations of the crown confined to the posterior border, or at all events much less developed on the front edge. Very little is known of the structure of these animals, beyond the skull and teeth, fragments of which have been found widely distributed throughout the marine Miocene and Early Pliocene formations of Europe, especially in the Vienna basin, many parts of France, and the Antwerp and Suffolk Crags. They have also been found in formations of corresponding age in North America and South Australia. Family PLATAXISTJD.E. Under this heading may be placed three very singular genera, which, though diil ering considerably from each other, have several points in common, and do not altogether come under the definition either of the Physctcridss or the . Delphinidse, especially in the important character of the mode of articulation of the ribs with the dorsal vertebra?, as the tubercular and capitular articulations, distinct at the commencement of the series, gradually blend together, as they do in most ordinary mammals. The cervical vertebra are all free. The lacrymal bone is not distinct from the malar. The jaws are long and narrow, with numerous teeth in both. The symphysis of the mandible exceeds half the length of the whole ramus. Externally the head is divided from the body by a slightly constricted neck. Pectoral limbs broad and truncated. Dorsal fin small or obsolete. Fluviatile or estuarine. There are three distinct genera, which might almost be made the types of families, but it is probably more convenient to keep them together. Platanista. Teeth about |$ on each side, set near together, rather large, cylindrical, and sharp-pointed in the young ; in old animals acquiring a large laterally compressed base, which in the posterior part of the series becomes irregularly divided into roots. As the conical enamel-covered crown wears away, the teeth of the young and old animals have a totally different appearance. The rostrum and dentigerous portion of the mandible are so narrow that FIG. 47. Platanifia ganyetica. From Anderson. the teeth of the two sides are almost in contact. Maxilhe support ing very large, incurved, compressed bony crests, which overarch the nares and base of the rostrum, and almost meet in the middle line above. Orbits very small and eyes rudimentary, without crystalline lens. External respiratory aperture longitudinal, linear. Vertebra? : C 7, D 11, L 8, C 25 ; total 51. A small caecum. No pelvic bones. Dorsal fin represented by a low ridge. One species, P. gangctica, entirely fluviatile, being extensively distributed throughout nearly the whole of the river systems, not only of the Ganges, but of the Brahmaputra and Indus, ascending as high as there is water enough to swim in, but never passing out to

sea. It is quite blind, and feeds on small fish and Crustacea,