very superficial manner in those parts of North America where they are well developed and finely exposed for the collection of fossils. This leads me to think that it is only a question of time and detailed work to bring to light a large and varied crustacean fauna. This almost certainly existed, as proven by the occurrence of Beltina in the pre-Cambrian of the Rocky Mountains.[1]
Bernard's very interesting and valuable study of Apus[2] is replete with suggestions and inferences bearing on the evolution of the Crustacea from a browsing carnivorous annelid with its first 5 segments (head) bent so that its mouth faced ventrally and posteriorly, and using its parapodia for pushing food into its mouth. He concludes that the modern representative of this crustacean-annelid is Apus. With Bernard's theory in mind I have examined the Burgess shale annelidan and crustacean fauna to ascertain if there was an annelid that could be considered as representing his hypothetical crustacean-annelid, and nearer to it in structure than Apus. I found specimens of Canadia spinosa Walcott laterally flattened in the shale with the head bent down, so that the mouth faces posteriorly,[3] also that 14 out of 24 specimens have the head bent under and out of sight beneath the flattened body. Possibly these annelids and the crustaceans were derived from the same general type of animal.
Among the crustaceans Marrella splendens (pls. 25 and 26) has an Apus-like form, but it is evidently a more highly developed form than Apus. This is shown among other characters by its carapace, long jointed legs, and fewer segments.[4] Burgessia bella (pl. 27, figs. 1-3) has a simple carapace, few thoracic segments, and many abdominal segments, if those of the telson-like extension of the body are considered as belonging to the abdominal region. The eight thoracic segments serve to separate Burgessia from Apus and other genera of the Apodidæ and at the same time bring it near to the Phyllocarida as represented by Nebalia. On the other hand, the simple Lepidurus-like carapace, sessile eyes, and hepatic glands in the
- ↑ C. D. Walcott, 1899, Pre-Cambrian Fossiliferous Formations ; Bull. Geol. Soc. America, Vol. 10, pp. 238-239, pls. 25 and 26.
- ↑ The Apodidæ. Nature Series, London, 1892.
- ↑ Smithsonian Misc. Coll., Vol. 57, No. 5, 1911, pl. 23, fig. 4.
- ↑ Dr. Austin H. Clark considers that a comparison should be made between Marrella and the Trilobita. He suggests that the cephalon is comparable with that of Acidaspis, the two anterior spines being the genal spines and the posterior spines the same as the occipital spines or processes of Acidaspis. The terminal plate he takes to be the pygidium, and the feathery organ (m) the last cephalic appendage.
