294 Organic Remains, ^'c. 



just observation : " I believe all geologists agree that approximatmg 

 strata pass into each other, and that alternations are made up of ele- 

 ments common to both. Hence it is that we often find much diffi- 

 culty in fixing on the line of division." I cheerfully grant this diffi- 

 culty in the present instance ; nor am I prepared to speak decidedly 

 on the subject. 



It is my intention, when leisure permits, to visit the marl localities 

 in the neighborhood of Amboy and Middletown Point j and to ex- 

 amine particularly the clay and lignite strata at Bordentown and White 

 Hill, both which I suppose to be the plastic clay formation resting on 

 the marls described in this paper. I design to embody these observa- 

 tions in a second part of this synopsis, which will also embrace noti- 

 ces of some additional fossils, which, from coming late to my hands, 

 have been unavoidably omitted. 



JVote. — Since the preceding paper was transmitted to Professor 

 Silliman, I have received the 104th number of Mr. Sowerby's Min- 

 eral Conchology, and it gives me much pleasure to observe that that 

 gentleman has adopted Mr. Say's genus Exogyra. On this subject 

 I shall make two quotations, and subjoin a few remarks. 



" The mistake," says Mr. Sowerby, " under which several shells 

 that might better have been called Gryphcese, were published in the 

 first volume of the Mineral Conchology, as Chamse, seems to have 

 arisen from considering the tooth in the hinge, without taking into ac- 

 count the muscular impressions, which in Chama are two. This im- 

 portant tooth seems to have been overlooked by Mr. Say, though he 

 mentions the cavity, or furrow, that receives it. In the sixth volume 

 of the Journal of the Academy of Natural Sciences of Philadelphia, 

 Dr. Morton, when describing the Exogyra costata of America, has 

 rightly referred the Chama haliotoidea (Min. Conch, t. 25) to the 

 same genus, and discovers the mistake above alluded to." 



Mr. Sowerby then transfers to the genus Exogyra, five fossil shells 

 (all found in, or below the chalk formation) which are described as 

 Chamae in his Mineral Conchology, and among them are the Chama 

 haliotoidea, and C. conica, which I had indicated as belonging to Mr. 

 Say's genus. 



With respect to the geological inferences to be deduced from these 

 fossils, Mr. Sowerby remarks as follows : 



" Exogyrffi appear confined principally to the green sand forma- 

 tion ; but as we do not know that the American species, E. costata, 



