536 THE PALEONTOLOGY OF MINNESOTA. 



[Cyrtodonta subovata. 



The above is a fair statement of the case as I found it when I began the 

 present work. Had my studies shown what both Billings and Hall conceded to be 

 the case, that Conrad's sketch of the hinge of Cypricardites was identical with that 

 of Cyrtodonta and Pala'arca,. I would most surely have sided with Hall and adopted 

 the oldest name. But here was the rub. Comparisons with the hinges of numerous 

 species of this family of shells have demonstrated beyond question that Conrad's 

 figure and description of the hinge of Cypricardites does not correspond exactly 

 with that of any true Cyrtodonta or Vanuxemia known. He represents the cardinal 

 teeth as diverging from the beak much as in a Lyrodesma and says that the anterior 

 one is the "largest and most prominent". Neither of these conditions is ever 

 present in Crytodonta. On the contrary the teeth are subparallel, and to be called 

 horizontal rather than radial, while the anterior one, if any can be so called, is the 

 smaller. Nor have I seen any Cyrtodonta with five cardinal teeth, the usual 

 number being three; two is not uncommon, but four is very rare. 



We are now confronted with the question, did Conrad correctly describe and 

 illustrate the hinge of his genus? This question can be determined only by a study 

 of the type of the genus. But here again we meet with trouble for of the sixteen 

 species originally referred to the genus only one, his C. curtus remains, the others 

 having proved generically distinct, being now referred to other genera. The genus 

 must then, if it stands at all, be based upon C. curtus. I do not know whether the 

 hinge drawn by Conrad represents that of this species or not. For the present we 

 must assume that it does, and further, until we know the contrary, it must be 

 accepted as correct. From this standpoint then it is evident that Cyrtodonta and 

 Cypricardites are not synomymous, and that both may stand for the present. I 

 would suggest that, however the question may be eventually terminated, Cypricar- 

 dites may for a long time to come serve as a convenient temporary receptacle for 

 those species which because they are insufficiently known cannot be definitely 

 placed into other genera. 



CYRTODONTA 



PLATE XXXIX. FIGS. 28, 20, 31-33, ? 30 ami '- V,. 



.Shell somewhat obliquely ovate, narrowest anteriorly. Dorsal margin short, 

 less than half the length of the shell posterior to the beaks merging gradually into 

 the uniformly rounded posterior margin, base gently convex, anterior end short and 

 rather narrowly rounded; outline distinctly concave between the anterior extremity 

 and the projecting umbones. Beaks incurved, umbones prominently rounded, 

 inconspicuous. A slight flattening of the surface between the umbonal ridge and 



