INVERTEBRATE PALEONTOLOGY. 231 



It will be observed that in none of these descriptions is it stated that 

 Poromya proper lias more than a single cardinal tooth in the righl valve, or 

 any well-defined teeth or tooth whatever in the left valve; while the pres- 

 ence of a small internal cartilage-pit is constantly mentioned. Nyst, who 

 first described the typical species of Poromya doubtfully under the genus 

 Corbula (Coq. et Polypiers FossilesTerr.Tert.de la Belgique, 71). from 

 Tertiary specimens, also says that "its hinge is provided with one tooth.'' 

 H. and A. Adams, who, as has been pointed out by Deshayes and Dr. 

 Stoliczka, wrongly make Poromya a synonym of Tln>tis, Sowerby (an extinct 

 genus, with which they were not well acquainted), describe the hinge, 

 apparently partly, at least, from the typical Poromya, as having " a single 

 erect cardinal tooth in the right valve, received into a corresponding fossa 

 in the left :" adding, also, that there are "no lateral teeth in the right valve, 

 but an anterior and posterior lateral tooth in the left ; ligament internal, 

 inserted in a socket in each valve."* Deshayes likewise describes the hinge 

 of Poromya, from a Paris-basin shell, as having "one conical sharp tooth in 

 each valve; that of the left valve being smaller and sometimes obsolete."! 

 His species, however, is a transversely-oblong, decidedly iuequi valve shell, that 

 appears to me to be very unlike the typical Poromya, and to have no analogy 

 whatever to our Cretaceous forms. 



In first proposing the names Liopistha and Gymella for two Cretaceous 

 shells, I was led to do so entirely from their peculiar external characters; 

 none of the specimens then known to me showing the hinge of either. Since 

 that time, thanks to my friend Mr. T. A. Conrad, of Philadelphia, I have had 

 an opportunity to study typical specimens of each, discovered by him, showing 

 the hinge-characters in the most satisfactory manner. By his permission, 

 I have had prepared the wood-cuts at the head of this description, showing 

 the hinges and exterior of these shells. From these it will be seen that in 

 both there are two well-dev"eloped cardinal teeth under the beak of the right 

 valve, and one equally well developed, with a smaller oblique or horizontal, 

 rudimentary second cardinal in the left, without any internal cartilage-pit in 

 either valve. Of the two cardinals of the right valve, the posterior one is 

 the larger and (in Cymella) compressed from above and below, being a little 



* Genera Recent M ill.. II, 367. Tin; lateral teeth thpy must have seen in some of the other types 

 that they cite ;i^ synonyms. 



tSupi'l. Coq. Foss. ilea Environs do Paris, I, 248. 



