Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/460

442 REPTILES [CLASSIFICATION. tory movements are slow and irregular. In consequence, Reptiles are cold-blooded animals. Their blood-corpuscles are red and nucleated. The thoracic and abdominal viscera are never separated by a complete diaphragm. The intestinal tract and the urogenital organs open into a common cloaca; the oviducts are developed from the Miillerian ducts, and dilated in their lower course for the reception of the ova; all Reptiles are oviparous or ovoviviparous. The vertebral column articulates with the skull almost invariably by means of a single convex occipital condyle. The mandible consists of several distinct pieces, of which the articular bone articulates with a quadrate bone, interposed between skull and mandible. When the appendicular parts of the skeleton are present, the sternum is never replaced by membrane bone, and the posterior sternal ribs are attached to a median prolongation of the sternum. The ilia are prolonged farther behind the acetabulum than in front of it; the pubic bones directed downward and forward, and, like the ischia, forming a median symphysis. The metatarsal bones are not anchylosed among them- selves or with the distal tarsal bones. As in Birds and Mammals, the foetus of Reptiles is enclosed in an amnion and allantois (Amniota), and nourished from the vitellus of the egg. In some of the most important characters mentioned above Reptiles agree with Birds, as in the presence of a single occipital condyle, a complex lower jaw articulated to the skull by a quadrate bone, and nucleated blood cor- puscles. The majority of naturalists, therefore, consider the two classes to constitute one of the main divisions of Vertebrates, the Sauropsida. At the present epoch, indeed, Birds are strikingly differentiated from Reptiles, but the discoveries within recent years of a number of extinct Birds with Reptilian characters offer ample evidence that Birds are the descendants of some branch or branches of the Reptilian type, in which the power of flight was developed, and with it other anatomical peculiarities by which Birds are now distinguished from living Reptiles. THE DIVISION OF REPTILIA INTO ORDERS. Of the various modifications that have been proposed in the classification of Reptiles the more important are mentioned in the historical part of this article. We adopt here a serial arrangement of those orders which seem to be well established, having already referred to the attempts that have been made to arrange these orders into higher groups. Order 1. ICHTHYOPTERYGIA (extinct). Marine Reptiles with a Cetacean-like naked body and with four limbs formed into paddles, the parts of which after the humerus are not differentiated as to form or function. Tail long. Vertebrae numerous, biconcave ; no sacrum. Dorsal vertebras with double tubercles; ribs movable, the anterior with bifurcate heads. Head large, with long powerful snout, joined to the trunk without neck. Quadrate bone immovably articulated to squamosal. A foramen parietale is present. Orbits very large with a ^'circle of sclerotic Flo. 1. Skeleton of Iguanodon bemissartensis (after Dollo). plates. A pair of clavicles rest upon an interclavicle and pass laterally to the scapulas ; a pair of broad not over- lapping coracoids form the posterior part of the pectoral arch. A sternum is replaced by a series of abdominal splints. Fam. a. Sauranodontidas. Edentulous. Genus : Sauranodon, from the Jurassic formations of the Rocky Mountain region. Fam. b. Ichthyosauridaz. Teeth numerous, implanted in a com- mon alveolar groove. Genus: Ichthyosaurus, from Mesozoic strata up to the Chalk. Order 2. ANOMODONTIA (extinct). Lacertiform Reptiles, the skull and four limbs of which are Lacertilian in most of their characters. Vertebrae biconcave, four or five of them anchylosed together and forming a sacrum. The tubercular and capitular articulations are separated, the former and longer being on the diapophysis, the latter and shorter on the centrum ; ribs movable, the anterior with a bifurcate head. Os quadratum suturally connected with the skull. A foramen parietale is present. Jaws Chelonian and probably cased in horny sheaths ; either edentulous or each maxillary bone was armed with a long evergrowing tusk, which sometimes was accompanied by other smaller teeth. The pectoral arch consisted of scapula and coracoid, but a clavicle seems to have been absent. Pelvis very strong, with continuous ischio-pubic symphysis. Genera : Dicynodon, Galesaurus. Order 3. DINOSAURIA (extinct). This comprises Reptiles of a great diversity of form and size, some adapted for a terrestrial, others for an aquatic life, some carnivorous, the majority herbivorous, but all distinguished by charac- ters leading more or less closely from the Reptilian up to the Avian type. The majority of trunk vertebrae have flat or slightly concave articular ends, sometimes a few of the anterior are convex in front ; cervical vertebras numer- ous; a sacrum is formed by more than two coalesced vertebras. Neural arches united to the centra by sutures. Thoracic ribs movable, with a bifurcate head; cervical ribs united to the vertebrae either by suture or anchylosed.