. Carnegie Institution of Washington publication. PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 69 detennined is that Sphenacodon is a typical pelycosaur, very closely allied to Dimetrodon. with only minor differences in skull, girdles, extremities, and \'ertebral centra, but with very much shorter spines, which are not pointed at their extremities, but, on the other hand, are dilated antero-posteriorly. The spines are long, extraordinarily long for a reptile, but they could not have fonned any such frill as that of Dimetrodon or the basilisk lizard. And this structure of the vertebra has a
. Carnegie Institution of Washington publication. PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 69 detennined is that Sphenacodon is a typical pelycosaur, very closely allied to Dimetrodon. with only minor differences in skull, girdles, extremities, and \'ertebral centra, but with very much shorter spines, which are not pointed at their extremities, but, on the other hand, are dilated antero-posteriorly. The spines are long, extraordinarily long for a reptile, but they could not have fonned any such frill as that of Dimetrodon or the basilisk lizard. And this structure of the vertebra has an important bearing in any discussion as to the meaning of the spines in Dimetrodon. That Dimetrodon could have developed such extra- ordinary spines without affecting to a greater degree the characters of the skeleton proves conclusi\'ely their relative physiological unimportance.* Certainly, had the enormous dorsal expansion of Dimetrodon been of profound importance in the life econom\' of these. Fig. 44.—Sphenacodon ferox Marsh. A, right scapula. No. 818, Yale University, X ]'i\ B, right scapula of a large Dimetrodon from Coffee Creek, Wilbarger County, Texas, No. 661, Yale University, X M- creatures, it must have materially affected the structure of the skeleton elsewhere. That Sphenacodon is more primitive than Dimetrodon must be admitted to be perhaps another bit of evidence of the greater antiquity of the New Mexico deposits than the upper ones, at least, of Texas. *Case would hesitate to indorse the statement that these elongated spines were physiologically unim- portant. He has long considered (and frequently stated his belief) that the enormous development of the spines in Dimetrodon and probably also in Edaplwsaurus (Naosauriis), imposed upon the creatures a ph3's- iological burden so great in its demands upon the energies of the individual, both for their original production and the repair of frequent injuries, that it was an important, if not the chief, cause of
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