A DESCRIPTION OF THE GENUS DIMETRODON, COPE. 21 



" The Long-spined Theromorpha of the Permian Epoch." 18 Referring to Dimetrodon he 

 says : 



" The huge neural spines formed an elevated fin on the back. In a medium-sized 

 specimen of Dimetrodon, incisivus, where the vertebral body is 35 mm. in length, the 

 elevation of the spines is 900 mm., or twenty and. a half times as great. The apex of 

 the spine in this species is slender, and apparently was flexible. The utility is difficult 

 to imagine. Unless the animal had aquatic habits, and swam on its back, the crest or fin 

 must have been in the way of active movements. Accordingly the spines are occasionally 

 found distorted at the union of the faces of fractures. The limbs are not long enough 

 nor the claws acute enough to demonstrate arboreal habits, as in the existing genus 

 Basilicus, where a similar crest exists. A very peculiar species has been described under 

 the name of Naosaurus claviger Cope. There the spines are not quite so elevated as in 

 the D. incisivus, but they are more robust, and have transverse processes or branches 

 which resemble the yardarms of a ship mast. In a full-sized individual the longest cross- 

 arms, which are the lowest in position, have an expanse of 260 mm., or ten and a quarter 

 inches, while the spine has about the height of 500 mm. (19.75 in.), the body being 60 

 mm. long. The animal must have presented an extraordinary appearance. Perhaps its 

 dorsal armature resembled the branches of shrubs then, as they do now, and served to 

 conceal them in a brushy or wooded region ; or, more probably, the yardarms were con- 

 nected by membrane with the neural spine or mast, thus serving the animal as a sail with 

 which he navigated the waters of the Permian lakes. A very singular character of the 

 spines in all the species is that they are hollow, as in Ccelacanth fishes, and that the 

 central cavity is not closed at the apex. 



" There is a well-preserved cranium of the D. claviger, but the muzzle is unfortunately 

 wanting. The median line rises forward so that the convexity of the top of the muzzle 

 is higher than the posterior parts of the skull, whose profile descends rapidly. This 

 throws the orbit far back, and gives the animal a peculiar appearance. JVaosaurus differs 

 from Dimetrodon in the transverse processes of the neural spines of the vertebrae. There 

 are three species, which differ as follows : 



Spines of vertebrae cylindrical distally ; transverse processes replaced above by tuber- 

 osities JSF. cruciger. 



Spines of vertebras expanded and compressed above. Palatine teeth large, forming a 

 pavement If. microdus (Edaphosaurus microdus Cope). 



Palatine teeth much smaller and more widely spaced N. claviger. 



" All these species are from the Permian formation of Texas." 

 In a paper by Baur 19 on the humerus of the Amniota, published in 1886, a few 

 remarks are made on the relationship of the Theromorpha and the Mammalia. Baur 



