PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



43 



ultimately distinguished by specific characters only, where the chain from ancestor 

 to descendants nowhere lacks a single link. No character is ever more than varietal 

 until it is fixed and isolated physiologically by heredity and time. 



One may be speculative and assume that Ophiacodon is really the beginning 

 of the double-arched phylum, the beginning of a subclass, but there is not the 

 slightest proof that such is the case. It will be many years before such an hypoth- 

 esis can be proven or disproven, if it were ever possible, and classifications based 

 upon hypotheses are unscientific. If the present form is merely a "mutation" or 

 sport, then certainly the presence of a supratemporal vacuity has no taxonomic 

 value, for we have no faith in the theory that species ever arise in such sudden 

 ways. In other words, the most profound taxonomic characters must have had 

 beginnings when they were merely varietal or specific ; not till time and long descent 

 have fixed them do they achieve a higher rank. In the present case we are con- 

 vinced that, without further evidence to the contrary, the presence of two temporal 

 vacmties and two temporal arches in Ophiacodon can not be assumed to have 

 more than generic value. 



Vertebra and ribs: As has been stated, the vertebral column as collected was 

 without break or even noticeable disturbance as far back as the sacrum, lying 



Fig. 24. — Ophiacodon mirus Marsh. Cervical vertebra; from the side, the sixth also from in front, X ^2. 

 ax, axis; an, atlantal neuropophysis; 0, odontoid; r, r, ribs; q, quadrate; pa, proatlas. 



with the skull and all or nearly all the ribs in natural articulation. Six or seven 

 of the vertebrae may properly be called cervical, and twenty or twenty-one dorsal ; 

 the entire number, as in Dimetrodon, Varanosaurus, Theropleura,* and probably 

 all other pelycosaurs in the narrower sense of the word, is twenty-seven in all 

 certainty. 



Proatlas (fig. 24, pa). A proatlas has never before been recognized in an Ameri- 

 can Paleozoic reptile. Its existence in the present genus was not unexpected, since 

 Williston has already suggested that the facets on the sides of the foramen magnum 

 in Dimetrodon are for the articulation of this bone.f In the present specimen 

 the proatlas lies quite in position, articulating posteriorly by a well-formed zyga- 

 pophysis on each side with the arch of the atlas, and in front on each side with a 

 facet on the margin of the foramen magnum. Because of a slight injury on the 

 right side, it can not be said positively that the element is single, though in much 



* Twenty-six presacral vertebrae back of the atlas are figured by Case in Theropleura retroversa (Publi- 

 cation 55, Carnegie Institution of Washington, pi. xill, fig. i). 

 t American Permian Vertebrates, p. 93. 

 4 



