VERTEBRATES. 413 



tlie middle and greatly deflected to the lateral extremities, though in a 

 much less degree than in P. unguiformis. Behind the coronal border 

 the basal area is gradually narrowed and produced into a proportion- 

 ately strong root, which exhibits the vertically excavated inner shoulder 

 and trowel-shaped inferior outline common to the genus. The coronal 

 surfaces are ornamented with vermiculose lines and punctae, much in 

 the same manner as in P. unguiformis. 



This form is represented by only one specimen, from the middle divi- 

 sion of the Coal Measures and exhibits only the convex aspect of the 

 tooth. It differs from that described from the Upper Coal Measures, 

 r. unguiformis, IS. and W., in its greater lateral extent as compared 

 ■with the anteroposterior diameter, also in the character of the coronal 

 fold in the convex face, which is not continued down the lateral margins 

 as far as in the last mentioned form. From. P. quadratus of the St. 

 Louis limestone, it is distinguished by its more oval outline and rela- 

 tively stronger root ; otherwise all the forms are intimately allied. 



Position and locality: In the limestone overlying coal No. 5 ; Belle- 

 ville, Illinois. 



Genus FISSODTTS, St. J. and W. 



Teeth small. Crown forming a comparatively thin concavo convex 

 plate together with the basal region, subcurvilinear or trapezoidal in 

 general outline, as seen from the concave side, the basal margin of 

 •which is produced or strongly arched and rounded from the ear-like 

 lateral angles, and bordered by an imbricated coronal belt ; convex face 

 relatively low, more or less uniformly convex with the basal region, 

 from which it is defined by a slight coronal fold, which describes a 

 broad arch outwards from the lateral angles and more or less curved 

 downwards in the middle ; crest more or less compressed, and deeply 

 cleft, or divided into two or more strong, acuminate, uniform, trenchant 

 cusps ; both coronal surfaces, in perfect state, enveloped in a dense, 

 polished enamel-like layer. The root originates in the gently convex 

 basal area just beneath the lower crown-face, proportionately small, and 

 produced in a thin, trowel-slraped process, which is somewhat deeply 

 channeled longitudinally in the region of its origin, narrowed and 

 truncated below. 



The teeth embraced under the above generic description comprise a 

 group closely allied to Peltodtis, which it resembles in the form of the 

 root and general contour, but from which it is distinguished by the cleft 

 condition of the crest — differences akin to those which distinguish Gten- 

 ojpetalus from Petalodus. It also presents, in the outline of the basal 



