74 REVISION OF THE PELYCOSAURIA. 



Table IV. — Contrasting the Characters of the Genera of the American Clepsydropid.?-— Continued. 



ClepsydropincE — Continued. 

 II. Dimetrodon. 



1. Large diastemal notch with few degenerate teeth or none. 



2. Canine and incisors greatly enlarged. 



3. Teeth with crenate cutting edges. 



4. Neural arch very early coossified with the centrum. 



5- Very marked change in the length of the bottom line of the centrum in different parts of the ver- 

 tebral column. Posterior lumbars shortened, but without wide faces on the lower edge of the 

 anterior face of the centrum. 



6. Anterior dorsals with a sharp median keel and wide intercentral face on the lower edge of the 



anterior face of the centrum. 



7. Spines sharply recurved in posterior lumbar and sacral regions. 



8. Spine of the axis high and broad, reaching forward over the axis. 



9. Limb bones with well-marked articular surfaces. 



10. Humerus with well-developed entepicondyle and ectepicondylar process sheltering an ectepicon- 



dylar notch. 



11. Abdominal ribs absent. (Not observed.) 



12. Tail short and stumpy. 



13. Size varying in the different species ; from about 1.2 to about 3 meters. 



(■^aosaurina. 



III. Naosaurus. 



1. Large diastemal notch with few degenerate teeth or none. (?) 



2. Canine and incisors greatly enlarged. (?) 



3. Teeth with crenate cutting edges. (?) 



4. Neural arch early coossified with centrum. 



5. Change in length of the bottom line of the centrum in different parts of the column not marked. 



6. Anterior dorsals without median keel or wide intercentral face. A process on the lateral edge 



of the centrum marking the position of the capitulum of the rib. 



7. Spines sharply recurved in the posterior lumbars so that the last long spine overhangs the 



abruptly shortened spines of the sacrals and caudals. 



8. Character not shown. 



9. Limb bones with the articular face well marked. 



10. Humerus with well-developed entepicondyle and ectepicondylar process sheltering an ectepicon- 



dylar notch. 



11. Abdominal scutes present. 



12. Tail short and stumpy. 



13. Size varying in different species. From 2 to 2.5 meters. 



Table V. — Contrasting the Characters of the Different Species of the Genus Dimetrodon. 



D. gigas. 



1. Spines quadrangular, nearly square at base; changing to rounded above. Large, the surface not striate. 



2. Third to the sixth cervical vertebrae with the axis through neural arch and the middle of the centrum ver- 



tical. 



3. Intercentra small in the mid-dorsal region ; without terminal facet for the capitulum of the rib. 



4. Character not shown. 



5. Size, largest of the genus. From 2.5 to 3 meters in length. 



D. giganhomogous. 



1. Spines broad transversely at base; becoming co -shaped above. A tendency for the ends of the spine 



to become tuberculate. 



2. The third to the sixth cervical vertebrae with the axis through the neural spine and the middle of the cen- 



trum vertical. 



3. Intercentrum in the mid-dorsal and lumbar regions small and without terminal facets for the capitulum 



of the rib. 



4. Character not shown. 



5. Size nearly as great as that of D. gigas, 2.6 meters at least. 



