INVERTEBRATE PAL2EONTOLOGY. 81 



and among the established sections of that group it seems to come nearest 

 agreeing with Mr. Conrad's Polynema. It is, however, a more depressed, or 

 proportionally more elongated, shell than Mr. Conrad's type, and less sin- 

 uous below, with its anterior ventral margin merely rounded up from the 

 base, instead of being produced and rounded, as in the type of Polynema; 

 which latter is also proportionally much wider (higher) behind. Still, I do 

 not think these differences of sufficient importance to place our shell in a dif- 

 ferent subgenus. 



Locality and position. — Twelve miles southwest of Salina, Kansas; in a 

 ferruginous sandstone, equivalent to the Dakota group of the Upper Missouri 

 Cretaceous. Collected by Professor Mudge. 



Genus NEMOBON, Conrad. 



Synon. — Area, Cucullwa and Maero&on (sp.), of Bome authors. 



Nemodon, Conrad (1S70), Am. Jour. Concli., V, 97; and (1ST: 1 ,) Appendix to Prof. Kin's Geol. 

 Rt-pt. N. Carolina, 3. 



Etym. — vfjiia, a thread ; bSovg, a tooth. 

 Type.— Nemodon Evfalensis, Conrad. 



Shell depressed, and transversely more or less elongate-trapezoidal, thin, 

 equivalve, with base sinuous and perhaps gaping ; beaks depressed and placed 

 in advance of the middle; cardinal area narrow ; hinge long, straight, or slightly 

 arcuate under the beaks, where it is very narrow and provided with a few 

 minute granular denticles, in front of which there are two or three linear an- 

 terior teeth very nearly or quite parallel to the cardinal margin, while behind 

 the beaks there are one or two elongate, linear, posterior teeth, also parallel to 

 the cardinal edge; surface marked by very obscure, punctate, radiating striae, 

 and concentric lines. 



This group consists of small, generally transversely-elongated, rhombic 

 shells, having much the general external appearance of Area and Macrodon.* 

 It is at once distinguished from the former, however, by having its hinge-teeth 

 elongated and parallel to the hinge-margin, instead of very short, mere crenu- 

 lations, crossing the same. It is much more nearly related to Macrodon, and 

 might, perhaps, without impropriety, be ranged as a subgenus under that 

 group, from which it differs chiefly in having its anterior teeth parallel to the 

 hinge margin, instead of ranging obliquely forward and upward, and in having 

 a few minute granular teeth under the beaks. Mr. Conrad does not say 



* The name Macrodon being pre-occupied for a genus of fishes, will probably have to be replaced by 

 Parallelodon, Meek and Worthen, proposed in the Proceed. Chicago Acad., I, 17, 1806. 



11 II 



