MARYLAND GEOLOGICAL SURVEY 629 



order ANOMALODESMACEA 

 Superfamily ANATINACEA 



Family PHOLADOMYIDAE 



Genus PHOLADOMYA Sowerby 

 [Genera Recent and Fossil Shells, 1825, pp. 235, 236, pi. xxxvii] 



Type. Plioladomya Candida Sowerby. 



" The following generic character being drawn up principally from the 

 recent specimen, several particulars will be mentioned in it which cannot 

 be observed in the fossils ; there is not, however, the smallest doubt as to 

 their generic identity. Shell very thin, rather hyaline, transverse, ventri- 

 cose ; inside pearly ; posterior side short, sometimes very short, rounded ; 

 anterior side more or less elongated, gaping; upper edge also gaping a 

 little ; hinge with a small rather elongated, triangular pit, and a marginal 

 lamina in each valve, to the outer part of which is attached the rather 

 short external ligament. Muscular impressions two ; these, as well as the 

 muscular impression of the mantle, in which there is a large sinus, are 

 indistinct. This shell is the only instance we have ever seen in which the 

 umbones are so approximated as to be worn through by the natural action 

 of the animal in opening and closing its valves." Sowerby, 1825. 



Equivalved or subequivalved, inequilateral, transversely elongated or 

 subtrigonal, gaping posteriorly and sometimes anteriorly as well ; umbones 

 inflated, anterior ; external sculpture radial, often more or less nodose ; 

 ligament short, external, opisthodetic ; cardinal margin often reflected to 

 form a false area behind the umbones ; hinge edentulous excepting a single 

 subumbonal tubercle and pit in each valve; muscle impressions obscure, 

 two in number ; pallial sinus profound. 



The genus was initiated early in the lower Lias, and though it culmi- 

 nated later in the Jurassic, the decline was not marked until the close of 

 the Mesozoic. The Tertiary representation, however, was very meager 

 and less than half a dozen species have persisted to the present day. As in 

 so many of the ancient types, the few survivors have retreated to unf avor- 



Etymology: A name suggested by " its resemblance to shells of two Lin- 

 nean genera, the Pholades and Mya." 



