Sand Formation of the United States. 245 



BIVALVES. 



PLAGiosTOMA. Sowerby. 



1 . From one to three inches long, with numerous delicate longitu- 

 dinal ribs, and elevated concentric squamous plates. Is found at- 

 tached to other shells in the same way th^t some British species are 

 attached to flints. 



2 ? Casts are found in the marl of the Chesapeake and Delaware 

 Canal resembling those figured by Sowerby as F. rusticum, pi. 381. 

 I am not certain, however, that the two fossils belong to the same genus. 



OSTREA. 



5. About an inch long, compressed, with numerous diverging, spi- 

 nous costag. A remarkably perfect fossil, from near Arneytown, N. J. 



PECTEN, 



3. Compressed, thin, longitudinally striated ; and bearing consid- 

 erable general resemblance to the chalk fossil P. nitidus of Sowerby, 

 tab. 394. 



4. A large species, found hitherto only in fragments. Costse large 

 and convex, with a smaller one intermediate. 



CLAViGELLA. Sowerby. 



A single specimen, apparently referrible to tliis singular genus, has 

 been found in New Jersey. 



ECHINIDE^. 



SPATANGUS. 



3. S. Stella. (S. G. M.) Small, globose, with pentapetalous 

 sulci ; the longitudinal groove does not reach to the base, in which 

 respect it differs from species 1st of this synopsis. Common in the 

 calcareous marls. It was first pointed out to me by Mr. Tifian R. 

 Peale. 



ANANCHYTES. 



2. A. cruciferus. (S. G. M.) Oval; less than an inch in length: 

 apex subcentral : the two lines composing each of the five pair of am- 

 bulacra are parallel throughout ; there is no sulcus. I refer this fossil 

 to ananchytes, although it does not in every respect agree with that 

 genus. Communicated by Mr. T. R. Peale. 



3. A.fimhriaius. (S. G. M.) Four pair of dotted ambulacra, with 

 eight or nine lines passing from the apex to the mouth, and a poste- 



