532 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 56. 



aspect of the skull differing chiefly in the proportions of some of its 

 elements. The anterior portion of the palatines are relatively wider 

 than represented by Broom, and the pterygoids, especially their 

 anterior prolongation, is clearly shown by this specimen to be nar- 

 rower than in any restoration yet given. It now appears quite 

 probable tliat in articulating this skull the palate especially between 

 the maxillaries has been made too narrow, thus closing up the 

 posterior median aperture. 



Vertebral 'column. — The complete backbone in front of the sacrum 

 in Dimetrodon, according to Dr. E. C. Case, consists of 27 vertebrae. 

 In the present specimen there were parts present of that number of 

 presacrals. The atlas was represented only by the preatlantal inter- 

 centrum, and vertebrae 11, 13, 14, and 15 (counting from the head 

 backward) by their spinous processes. The other vertebrae of the 

 series are unusually well preserved, but all except the fourth and sixth 

 lack the proximal terminations of the spines. In articulating the 

 skeleton the fourteenth and fifteenth have been assigned centra 

 that were found associated with this specimen, but there is no evi- 

 dence to show that they belong to these particular vertebrae. There 

 is a single caudal present that on account of the large size of its 

 transverse processes is called the fu'st of the series. The missing 

 sacral vertebrae have been supplied from a second individual (No. 

 8661, U.S.N.M.), belonging to the same collection and from the 

 same deposit of fossils. It appears to pertain to a slightly smaller 

 individual than No. 8635, U.S.N.M. With the exception of the 

 single caudal mentioned above the remainder of the tail has been 

 restored. 



The fin-like crest along the back formed by the lengthened spinous 

 processes of the vertebrae presents a notably different aspect than 

 found in the previously mounted skeletons of Dimetrodon incisivus, 

 but whether the differences observed represent constant features 

 peculiar to Dimetrodon gigas, I am not prepared to decide at this 

 time. Unlike D. incisivus, where the spines rise perpendicularly 

 and rapidl}^ lengthen from the axis backward, the first four posterior 

 to that bone in this specimen are of about equal height. Beginning 

 with the seventh of the series they rapidly grow longer as we pro- 

 ceed posteriorly in the column, reaching their maximum develop- 

 ment with the twelfth vertebrae. Continuing posteriorly they 

 gradually diminish in length, finally graduating into the short spines 

 of the tail. The processes of the first dorsals have a tendency to 

 rise vertically, but as we pass backsvard they assume an oblique and 

 finally a curved retroverted position overhanging the sacrum. 



The spinous process on the fourth cervical is complete, being 

 ternunated by a cupped extremity. It measures 220 mm. in length 

 from center of neural canal to tip. The sixth spine is very nearly 

 perfect, lacking only a small portion of the tip. All of the others 



