PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



47 



the wash of the two specimens numerous other caudal vertebrae were obtained, 

 but it is impossible to say to which skeleton they belong; in all probability they 

 belong with both, though there seem to be few or no duplicates. As they differ so 

 little in their characters, only a few have been figured — the four distal ones shown 

 in fig. 27 B. The first one shown in fig. 27 a may be either the third or the fourth 

 of the series. The tail as a whole must have been very much like that of Vara- 

 nosaurus, long and slender, and probably included, as in that genus, fifty or more 

 vertebrae. Conspicuously different, however, are the numbers of pygal vertebrae, 

 that is, those without chevrons, in the two genera. In Varanosaurus there are not 

 more than three, while in Ophiacodon there are at least six, and possibly seven. 

 The spines decrease rapidly in size and height, the first complete ones preserved, 

 those of the eighth and ninth caudals, being even shorter than the corresponding 

 ones of Varanosaurus. Between the two series shown in figs. 27 a and B there is, 

 in all probability, but one vertebra missing, the perfect spine shown being that of 

 the fourteenth or fifteenth vertebra. 

 Between the last of this series and 

 the next one shown in the figures, 

 probably the thirtieth, a dozen ver- 

 tebrae are preserved. The smallest 

 vertebra figured is probably the 

 fortieth or thereabouts, possibly the 

 forty-fifth. 



From all of which it is quite cer- 

 tain that the tail was long and slen- 

 der, perhaps more slender than that 

 of Varanosaurus, of which forty- 

 seven caudal vertebrae have been 

 found in a continuous series, with 

 at least a half dozen terminal ones 

 missing. There are ten or more 



pairs of sutiu-ally united ribs, the proximal ends of many of which are yet connected 

 with their vertebrae, but whose lengths can not be determined. This is about the 

 number occurring in Varanosaurus and most other contemporary reptiles. The ante- 

 rior caudal centra are rather sharply keeled below, but the tmder side soon becomes 

 fiat, or has a pair of grooves separated by a low keel. Evidently well-developed 

 chevrons were present throughout the series back of the sixth or seventh, though 

 only a few fragments of them have been recovered. The tail very clearly was not 

 natatorial in structure. 



Ribs: All the vertebrae as far back as the eleventh or twelfth caudal are costif- 

 erous. The ribs have the usual articulations of the primitive reptiles, the head 

 resting more or less in the intercentral space, the tubercle articulating with the 

 arch. In no presacral vertebra is there a distinct parapophysial surface on the 

 centrum, but the tubercle articulates with a more or less prominent diapophysis. 

 In Ophiacodon there is a more or less continuous articular surface between the 

 capitulum and tuberculum, as in most of the contemporary cotylosaurs. In most 

 of the ribs this intervening surface is narrow, and in some it is obsolete, but in none 

 is there a distinct emargination as in other known pelycosaurs. 



In many of the primitive reptiles, such as Diadectes, Lahidosaurus, Capto- 

 rhinus, Pareiasaurus, Paleohatteria, and Sphenodon, as also the "Microsauria," 

 the ribs are commonly called single-headed, but incorrectly, since all have the two 

 articular surfaces, capitulum and tuberculum, the former articulating in or near 



Fig. 27. — Ophiacodon mirus Marsh. A, proximal caudal 

 vertebrae from side; B, proximal and distal caudal ver- 

 tebra from side, X K. 



