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Show 214 DR. R. BROOM ON THE [Mar. 21, developed independently in the two groups or that the groups had a common ancestor in which most of the characters were present. One or two may have originated independently, but it seems much more reasonable to assume that the two phyla sprang from an early true Reptile, than that they originated from a Batrachian and that all the characters they have in common have been developed independently. To what order the common reptilian ancestor belonged it is difficult to say. The Cotylosauria of Cope is no doubt very primitive, but unfortunately it is very imperfectly known. The type genus on which it was founded is Diadectes, a form which is not very well known, and the skull of which has so far not been very satisfactorily figured. A considerable number of other genera have been placed in the order by Cope and others, some at least of which are possibly not very nearly related to Diadectes. Pario-tichus, which is placed in the same order, is much better known, through the researches of Cope and, more recently, of Case. It has a skull roofed as in the Labyrinthodonts, but a palate very much like that of Sphenodon. But while Pariotichus might perhaps have been the common Amniote ancestor, so far as the skull is concerned, the condition of the shoulder-girdle shows that it is not primitive enough. It has lost the cleithrum which the ancestor must have retained. Pareiasaurus, though it retains the cleithrum, is further removed from the ancestral type than Pariotichus, but in another direction. It seems probable, however, that there were forms somewhat resembling Pariotichus but sufficiently primitive to have been the ancestor of Pareiasaurus. If such a form is discovered, then we may regard the Cotylosauria as containing the common ancestor of all reptiles. The phylum which contains Pareiasaurus, Dicynodon, the Theriodonts, and which culminates in the Mammals, probably originated through the ancestral Cotylosaurian living in marshy regions and having to walk with the body well supported off the ground. This habit caused the ilium to become directed mainly upwards and forwards from the acetabulum, and necessitated the retention of the precoracoid. From the start once given in this direction, the evolution went on steadily till the mammal was formed. The other phylum, which gave rise to the Lizards and Birds, was probably started by some of the early Cotylosaurians having to stalk insect prey on dry sandy or rocky places. The body rested for the most part on the ground and the legs became relatively feeble. As this was probably no very great change from the newt-like gait of the ancestor, it was long before there was much change in the structure. And some reptiles which are well advanced along the Diapsidan line, such as Mesosaurus or Palceo-hatteria, still retain the early types of shoulder-girdle and pelvis with very little modification. In Palceohatteria the now useless precoracoid ceases to be ossified, and in all the later Diapsidan types there is no trace of a precoracoid bone. The plate-like |
