PHOLADOMYA. 



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, ventricose ; 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. 



The general aspect of this shell, is between that of 

 Pholas and Aiiatina of Lamarck, but most of the fossil 

 species have been arranged as LutraricB. We have called 

 it Pholadomya, with reference to its resemblance to shells 

 of two Linnean genera, the Pholades and Myce, It is re- 

 lated to PanopdBii in the characters of the hinge, but may 

 be distinguished from that Genus by its thin, semitrans- 

 parent, pearly shell : from Pholas and Anatina, by its ex- 

 ternal ligament, and its want of external and internal ac- 

 cessary valves: and lastly, from the Lamarckian MycE^ 

 by not having the unequal teeth of that Genus. 



The fossil species of this new Genus, are represented 

 in Sowerby's Mineral Conchology, t. 197, 225, 226, 227, 

 297, and 327, under the names Cardita? producta, obtusa, 

 lyrata, deltoidea, and margaritacea; and Lutraria ly- 

 rata, ovalis, ambigua, and angustata. They occur in 

 several rocks of the Oolitic series, particularly the Corn- 

 brash, Inferior Oolite, and Fuller's Earth ; as well as in 

 the LiaS, the London Clay, and the Sutherland Coal-field : 

 also in the dark coloured Clay, between the Chalk Marl 

 and the Plastic Clay ? at Alum Bay. 



Our plate represents four views of Pholadomya can- 

 dida; test^ transversim oblong^, postice brevissim^, ro- 

 tundat^ ; mediant parte striis divaricatis, decussatis, ab 

 umbone decurrentibus ; antice elongate, subquadrata. 



