90 PALJ^ONTOLOGY OF NEW JERSEY. 



posterior extremity pointed and the posterior margin very obliquely trun- 

 cate, straight on the margin or slightly concave, hut on many of the casts 

 appearing rounded from the wearing away of parts, or perhaps from the 

 thickening of the shell on the inside. Surface of the casts showing indica- 

 tions of plications or ribs which when well-preserved appear to have been 

 strongest near the posterior part of the body of the shell, and gradually 

 decreasing in size until they become fine striae on the anterior end. Their 

 form, posterior to the umbonal ridge, cannot be determined fi'om any of the 

 examples examined. Muscular imprints large, but faintly marked; the ridge 

 bordering the posterior one not strong and scarcely indicated, but some- 

 times represented by a broad, shallow furrow in the cast. 



The specimens which I have identified as belonging to this species do 

 not agree in all particulars with the description as given bv Mr. Gabb; but 

 it is the only one known in the New Jersey beds which \\ ill ut all approach 

 it that has not been figured by the authors of the species. The principal 

 diffei'ence consists in the surface stria?, which he says are "numerous." This, 

 one would naturall}^ interpret as fine, but on the best preserved examples 

 which I have examined they are broad toward the umbonal slope, measur- 

 ing nearly a line in width. It is possible they may have been double, but 

 there are no indications of it, and on shells of this class, where the substance 

 has been very thick, the casts are often marked by a series of vascular lines, 

 which appear as radiating strife, but which have no immediate connection 

 with the external stria?, and it may have been these which he refers to. The 

 internal muscular plate he says is "low and broad." On the example bor- 

 rowed from the collection of the Academy of Natural Sciences of Philadel- 

 phia, this is the case, and I suppose it to be the example used by Mr. Gabb, 

 although found in the collection without name. From A. quindecimradiata 

 he says it differs in having "probably twice the number of ribs and by being 

 more convex." I have seen quite a number of examples of the form he 

 figures under that specific name, but they do not retain the markings of the 

 surface, but are marked with several strong, distant vascular ridges; but 

 these are not repi'esentatives of the surface stria?, and so far I have no evi- 

 dence of what its surface may have been. Besides, they are usually more 

 convex; that is, the valves are deeper according to the size of the shell than 



