138 



REVISION OF THE PELYCOSAURIA. 



The vertebrcF preserved are all of one type. They are short and high with the 

 lower end broad and marked by a wide groove with sharp edges, so the vertebra has 

 the appearance of being double keeled ; this is very different from the double keel 

 described in Dimctrodon doUovianus^ for there it is a sharp narrow keel and the groove 

 dividing it is very narrow. No vertebrce of this type have been found with any other 

 species of Dimetrodo)i ; the}' are much closer to the kind found in Theropleura^ and 

 might well belong to such an anin:a], having become accidentally associated with the 

 typical Dimetrodon humeri. On the other hand, no vertebrcC of the type of Dime- 

 trodcvi occur in the abundant material from New Mexico, and it seems probable that 

 the vertebrae must belong with the humeri. As we do not know the humerus of 

 T/ieroplettra, and as it is not certain that these vertebrae had high spines, it is, of 

 course, possible that D. navajovicits may turn out to belong close to Theropleiira. 



There are several fragments of small skulls which may be shown later to belong 

 here, as Archeobelus vellicaliis^ or two small maxillar}- fragments, Nos. 1013 and 1014 

 University of Chicago, from Texas. 



The Jmnieriis is 92 mm. long ; the proximal end 46 mm. wide, and the distal 

 end 70 mm. wide. 



T\i& front foot : A small front foot. No. 2290 Am. Mus., from New Mexico (plate 

 5, fig. 6), may possibly belong to this species. It resembles in almost every particular 

 the front foot of D. iiidsiz'iis, No. 1003 University of Chicago. This foot was described 

 by Cope (70) (plate 3, fig. 6) as posterior, but this mistake was largely due to the fact 

 that the radiale and centrale were separated from the rest of the foot and were not 

 recognized by him in the uncleaned material. 



Subfamily NAOSAURINAE Case (p. 5s). 

 Genus NAOSAURUS Cope, 



There is no certain evidence of the skull of Naosaurus ; according to Cope's 

 detenninations the skull is not distinguishable from that of Dimelrodon^ but, as 

 explained above (p. 58), the identifications can not now be confirmed either by the 

 specimens or records. (See note on p. 145.) 



The axis and atlas are unknown. The 

 cervical vertebrae show a most remarkable dif- 

 ference from those of Diinetrodon\ from the 

 fifth or sixth they decrease in size forward and 

 the spines are at the same time enlarged. In 

 Naosaurus claviger the section of the base of 

 the spine of the anterior cervicals is greater 

 than the centrum. In both Naosaurus microdus 

 and Naosaurus claviger the distal ends of the 

 neural spines of the cervicals are clavate, and 

 attach- this was probably true of A^. cruciger as well. 

 The details of the column are given in the 

 description of N. microdus. The bending of 

 the spines to the rear, which begins in the 

 anterior dorsal region, and becomes so acute in the presacral region, is most remarkable 

 and was at first attributed to accident, but as it occurs so regularly and in three dis- 

 tinct specimens, all that show this portion of the column, it may be accepted as natural. 

 Beyond a few anterior caudals the tail is unknown. 

 The clavicle, No. 4037 Am. Mus., is bent at nearly a right angle in the middle. 



^cl 



Clavicle of Naosau- 

 sp.. No. 4037 



TUS 



Am. Mus. 

 cl, surface f( 

 ment of cleithrum. 



Vz. 



