same individual of D. gigas has no head, but a regular wide crescentic 

 proximal articular surface. Below this on the posterior side is the large 

 trochanteric fossa, which is bounded by lateral ridges, which are at first 

 equal, but one soon exceeds the other in height, forming a trochanteric 

 ridge a little above the middle of the shaft. The condyles are distinct 

 from each other and are flattened below. One of them bears a robust 

 longitudinal crest above, which makes it much larger than the other, and 

 causes the groove that separates them above, to look outward, or to the 

 side which supports the trochanter. 



Three of the species may be distinguished as follows : 



Vertebral centra much compressed, acute below ; neural spines without 

 processes D. incisivus. 



Vertebral centra less compressed, obtuse below ; neural spines without 

 processes ; larger D. gigas. 



Vertebral centra compressed, not acute below ; neural spines with cross 

 projections D. cruciger. 



DlMETRODON CRUCIGER Cope. 



American Naturalist, 1878, p. 830. 



This species is not uncommon in the Permian Formation of Texas. It is 

 characterized by the enormous length of the neural spines of the lumbar 

 vertebrae, which form the dorsal fin seen in other species of the genus. 

 They are found in masses adhering together like sticks or branches of 

 bushes. In this species the spine sends off, a short distance above the 

 neural canal, a pair of opposite short branches, forming a cross. At vari- 

 ous more elevated positions there are given off tuberosities which alternate 

 with each other. They form on several consecutive spines oblique rows. 

 The spines are broadly oval in section, the long axis antero-posterior, and 

 have a shallow groove on both the anterior and posterior aspects. The 

 centra are elongate as compared with their other diameters, and are much 

 compressed between the articular extremities, leaving a strong inferior 

 median obtuse rib. Articular faces ot zygapophyses oblique. Diapophy- 

 ses short and robust, with large costal faces, and standing below the prezyg- 

 apophyses. 



Measurements. M. 



( antero-posterior 043 



Diameter of centrum ■} vertical at end 028 



(transverse at end 030 



Elevation of posterior zygapophyses above centrum 025 



" cruci form process " " 058 



Expanse of posterior zygapophyses 034 



" cruciform process 048 



-p.. „ . , . (antero-posterior ,030 



Diameter of spine at base i ^ .„_ 



(.transverse .020 



,, nnn i . f antero-posterior 016 



" " .090 above base j nin 



t transverse 01b 



Length of several pieces of neural spines 140 



