42 PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



In Dimetrodon Case has asserted the presence of an upper vacuity, but the evi- 

 dence is now apparently conclusive, and is so accepted by him, that, in some speci- 

 mens at least, there is no upper vacuity; and Williston, not having studied the 

 matter thoroughly, is not in a position to express a positive opinion as to its normal 

 absence, though he thinks such is the case. It will, perhaps, require additional 

 specimens of Ophiacodon to prove its constant occurrence in this genus, but it is 

 our firm opinion that the opening in the present specimen is a normal character 

 of the genus; it certainly is of this specimen. And this is said with a full appre- 

 ciation of the reputed fenestras in the skulls of Procolophon and Dicynodon, later 

 shown to be errors. 



As a bit of philosophy we may say that observers are often led into the recog- 

 nition of characters that accord with preconceived opinions. If one finds many 

 characters agreeing in different forms, he is naturally disposed to assume that 

 other characters do also. For a long while Procolophon was supposed to be closely 

 related to the rhynchocephalian reptiles — a supposition now summarily disposed 

 of, we think — to such an extent that it was located with the "Diapsida" and 

 ' ' Diaptosauria " even. But Sphenodon and the Rhynchocephalia have been respon- 

 sible in the past for many taxonomic sins, and we have not yet quite escaped from 

 the shadow of their misleading influence. 



In the present reptile the presence of a supratemporal fenestra was wholly 

 unexpected, and its discovery was rather a shock, since it seems almost hopelessly 

 to confuse the taxonomy of the Reptilia. That there is such a fenestra in the 

 present genus is, we think, beyond reasonable doubt. What is its significance? It 

 will be seen that not only the upper vacuity, but the lower one also, is remarkably 

 small, smaller proportionately than is known in any other double-arched reptile. 

 Are they rudimentary ? Or vestigial ? Is their small size due simply to the remark- 

 able development of the facial part of the skull without a corresponding develop- 

 ment of the posterior part ? Against this explanation is the fact that the temporal 

 region of Ophiacodon is, for the most part, covered by a broad expanse of bone, 

 necessitating the assumption that in the evolution of this particular type of skull 

 from some smaller one having proportionately larger vacuities everything has 

 developed except the vacuities. This hypothesis we think may be rejected. There 

 seems to be but one conclusion — that the openings are rudimentary. 



The presence of one or two temporal vacuities has hitherto been regarded as 

 a sort of fetich in reptilian taxonomy and phylogeny — as a crucial test of relation- 

 ships, not only of ordinal but even of subclass value in classification. For some 

 years past they have been considered as a sort of noli me tangere character, in that, 

 whatsoever of iconoclasm is permissible in regard to the feet, limbs, girdles, and 

 vertebras, hands must be kept off the sacred temporal region of reptiles. But 

 if we do not invade the sanctity here we shall be led into a dangerous morass of 

 speculation. In all other respects Ophiacodon is not only a "synapsidan" reptile^ 

 but a primitive one at that. To separate the form ordinally and classify it with 

 the "Diapsida" would be taxonomic speculation run wild. 



Could we know that Ophiacodon is the beginning of a morphological phylum 

 that ended in the Archosauria or Rhynchocephalia we would be quite justified, 

 notwithstanding all of its other and intimate relationships with the one-arched 

 or synapsidan forms, in separating it as the representative of a distinct order 

 of reptiles. In any true phylogenetic classification the lines of descent must be 

 followed back to where they diverge, to where the branches ultimately are sepa- 

 rated by nothing more than specific characters, since in the ideal classification of 

 animals and plants the truest is that in which orders, famiUes, and genera are 



