:VM-EXf!T.\ 
fc l) Di.tKX US B REV l A tiC A . ( ' d ' lirCid . 
Shell comparatively short ; hinge area minutely striated across, hinge 
line straight in the middle, descending towards the ends ; cardinal teeth 
minute, crowded ; sculpture, minute radiating lines. . , : 
This snbgenus allies Tpjgonoakca with Axjnjsa, and may readily be 
distinguished from either. by its minute close teeth. 
T. umbonata, pi. 1, fig. 8. Shell triangular, direct, equilateral, with 
a very prominent broad umbo and a terminal 'mnbonal slope which- is 
rounded ; posterior slope very small and almost at right angles with the 
disk ; lower half of disk flattened on the posterior side; senpture minute 
cancellated lines. 
T. perovalis, pi. 1, fig. 8. Shell oval, compressed, subequilateral, 
thin in substance ; margins rounded ; base mediately truncated ; mnbonal 
slope obtusely rounded ; 2 or 8 minute plications, near the beak on the 
anterior submargin ; inner margin entire. 
T. Carolinensis, pi. 1, fig. -1. Shell snbtriangular, inequilateral, veil- 
tricose, posterior margin subtruncated ; mnbonal slope undefined ; inner 
margin entire. . „ o. ■ * 
This species is shorter and more ventrieose than the preceding. 
T. congesia, pi. I, fig. 2. Shell rounded, ventrieose, slightly oblique, 
inequilateral, mnbonal slope undefined, 'margins rounded; cardinal series 
long, inner margin entire. ' • • 
This is perhaps the most abundant bivalve at Snow Hill, and yet in 
almost every instance consists ,of separated valves,, ..while in the genus 
Cvclotityris whole shells are not uncommon. 
T. S' afford l i, Gabb, is a member of this subgenus. 
SEMODObT. Conrad. 
This genus differs widely from the Jurassic Macrodon in having a very 
slender cardinal plate except towards the extremities, in having a few 
oblique linear teeth under the anterior side of the beaks, and in having 
the anterior teeth parallel with the hinge. margin. It differs from CtretiL- 
laeia, Deshayes, as represented by Area helerodonta, in having a simpler 
form of hinge character and a straight hinge margin. It comprises two 
forms of extremely thin shells, punctato-radiate on the exterior. 
