20 THE HISTORY OF THE PELYCOSAURIA, WITH 



especially of tlie anterior leg. It remains a fact that with this resemblance in the leg 

 there is a general adherence to the reptilian type in the structure of the skull." But this 

 adherence is not so exclusive as has been supposed, as he endeavors to show. 



An account is now given of the structure of the columella auris in Clepsydrops lepto- 

 cephalus. The columella resembles a rib, of which the suprastapedial process resembles 

 the head, and the stapes the tubercle. If this process be the incus, the stapes is short- 

 ened as in the majority of Mammalia. We have here an approximation to the Mammalia 

 in two points : (1) The perforation of the head of the stapes ; (2) and the ossification of 

 the incus, which (3) is distinct from the malleus, thus furnishing homologues of the prin- 

 cipal ossicles of the ear. 14 



The structure of the quadrate bone in the genus Clepsydrops is then discussed. This 

 bone in Clepsydrops leptocephalus Cope, already described, is of highly interesting form. 

 Its lower horizontal process is homologized with the zygomatic process of the squamosal 

 bone of the Mammalia, forming with the malar bone the zygomatic arch. " In the Pely- 

 cosauria there is but one posterior lateral arch, as is demonstrated by many specimens ; 

 hence, we have here a reptile with a zygomatic arch attached to the distal extremity of 

 the quadrate bone." 



After this some remarks follow about the articulation of the ribs in Embolophorus. 

 " The ribs of the Theromorpha are two-headed. While the tubercular articulation has 

 the usual position at the extremity of the diapophysis, the capitular is not distinctly, or 

 is but partially indicated, on the anterior edge of the centrum, in Clepsydrops and 

 Dimetrodon. In Embolophorus, as shown in 1878, the capitular articulation is distinctly 

 to the intercentrum." Therefore the ribs of the Theromorpha are intercentral and not 

 central elements, and are homologues, according to Cope, of the chevron bones [ ! ] . 

 This type of rib articulation also approximates closely that of the Mammalia, where the 

 capitular articulation is in a fossa excavated from two adjacent vertebrse. 



Finally the origin of the Mammalia is discussed. The Mammals are considered as 

 the descendants of the Pelycosauria, and a table shows the relations between the Amphibia, 

 Pelycosauria, other Reptilia, and the Mammalia. The same results were published in 

 the Proc. Amer. Assoc. Adv. Science, 15 Vol. xxxiii, pp. 471-482, 1 PI., Salem, 1885, with 

 the title, " The Relationships Between the Theromorphous Reptiles and the Monotreme 

 Mammalia." In April, 1885, Prof. Cope 16 published a paper "On the Evolution of the 

 Vertebrata, Progressive and Retrogressive." Here he derives all Reptilia, with the pos- 

 sible exception of the Ichthyosauria, from the Theromorpha. 



In April, 1886, Cope 17 gave figures of the vertebrse of Clepsydrops natalis Cope. In 

 June of the same year he established the genus Naosaurus, in a paper with the title, 



