FROM THE TERTIARY OF VIRGINIA. 235 



I believe, the first species of the genus as yet discovered in a fossil state in this country. 

 Mr. Conrad mentions having found the Fistulana elongata, Lam. at Claiborne, but, upon 

 comparison with M. Deshayes' figure of that shell, I do not think that it can be identical 

 with the present species. It is larger, more twisted, and has a different sculpture on the 

 anterior part. 



The genus GastrochaBna, as formed by Lamarck, was placed among the Pholadidse, 

 and considered to have no tube. M. Deshayes, however, includes it in the genus Fis- 

 tulana, as it inhabits a tube and has no teeth. Mr. Sowerby, again, considers as Gas- 

 trochsenaB those species which have a bent tube and an oval shell. According to this 

 arrangement, the present species would, most probably, come under this genus, and as 

 such I have placed it. 



There are two Gastrochsenaa among the fossils of Great Britain, one from the Oolite, 

 the other from the London Clay. Deshayes describes five from the Paris Basin. 



FAMILY PHOLADIDvE. 

 GENUS PHOLAS. 



P. RHOMBOIDEA, PL. 34. FlG. 7. 



P. testa transversd, oblique rhomboided. vald inaquilaterali, postice rotundato-angulatd, antice rostratd, inflatd, tenui, 

 media divisd lined impressd; area posticd striis lavibus conccntricis; area anticd striis acutis flexuosis concentricis; margine 

 basali curto; margine dorsali recto, valde reflexo; palmuld parvd, recurvd, tenui; lineis internis duabus elevatis longitudi- 

 nalibus. 



Shell transverse, obliquely rhomboidal, very inequilateral, rounded-angular posteriorly, rostrate anteriorly, inflated, 

 thin, divided in the middle by an impressed line; posterior area with smooth concentric striee; anterior area with 

 acute, flexuose, concentric striee; basal margin short; dorsal margin straight, very much reflected; palmule small, 

 recurved, thin; two longitudinal internal, elevated lines. 



Diam. .20 of an inch. Length .28. Breadth .18. 



Remarks. The strias on the anterior part of the shell are sometimes wrinkled so 

 regularly that they give it the appearance of being cancellate, although there are no 

 radiating lines. The posterior part of the shell is nearly smooth, from the dividing line 

 to the umbonial slope, where there are numerous regular lines of growth. There is a 

 kind of second dentiform process, immediately below the beaks, formed by the recurving 

 of the upper termination of the posterior margin. The two internal stria? extend from 

 the beaks to the base. The posterior one is situated immediately under the umbonial 

 slope. The anterior one is caused by the impressed dividing line. It extends, in a 

 point, a little beyond the basal margin. 



This shell is very similar to a specimen of the P. ovalis, Say, without the tube, and 

 with the thin anterior basal portion removed, but I believe it to be a perfect shell, as I 

 obtained it myself from the hole which it had formed, and in which it appeared to have 

 remained undisturbed. It was surrounded by a little indurated mud which preserved 

 it entire. It seemed to be entirely destitute of a tube, which character separates it from 

 the P. ovalis. It also differs in the shape and size of the palmule, and in the two in- 

 ternal lines. 



