LAMELLIBEANCHIATA OF THE PLASTIC CLAYS. 23 



mig'lit possibly resemble them in some cases, but not strongly. All these 

 forms found in the clays are estuary shells, and strongly indicate an estuary 

 formation, which it undoubtedly is. The generic characters of all of them 

 are obscure and cannot be positively affirmed, as their interiors are unknown. 

 They would seem to represent Gnathodon, Astarte, and Corhicula; and the 

 one most common, a new" genus which I have called Ambonicardia^ but none 

 of these would serve to definitely fix the geological horizon of the beds in 

 which they occur without other evidence. GnatJiodon is known to occur in 

 the Cretaceous, but I think not below, some of the older species having 

 been proved to belong to the genus Bangia, Astarte is known in the Juras- 

 sic and possibly below, but most of the Carboniferous species have been 

 shown to belong to other genera ; while Corhicula certainly occurs in the 

 Cretaceous and probably below. The new genus, which I have named 

 A^nbonicardia, is related to Homomya and to the smooth forms of Phola- 

 domya, but it will not answer for either. Consequently we get no help of 

 sufficient value to establish the geological horizon of the beds from these 

 moUuscan remains, and aside from the evidence furnished by the plant 

 remains we must rely entirely upon the stratigraphical position. 



In looking over the forms of Lamellibranchiate shells from the Jurassic 

 formations in the vain effort to find, some .genus under which I could class 

 this new form, I have been thoroughly impressed with the feeling that it was 

 more intimately allied to the Jurassic forms than to any of those from the 

 European Cretaceous. And I have a feeling, resting upon this evidence 

 alone, that the beds in which they occur represent that period on our east- 

 ern border. But the evidence is not sufficient to give basis for a positive 

 opinion. 



ASTARTIDJE. 



Genus ASTAETE Sowerby. 



Astarte yeta. 



Plate II, Fig. 1. 



Astarte veta Conrad. Am. Jour. Conch., Yol. lY, p. 279, PL XX, Fig. 4 (Fig. 5 by error 

 on descriptions of plate, see Yol. Y, p. 227). 



Mr. Conrad describes this species as ^^ Ovate, from ventral margin to 



