26 



The tentacula resemble those of phaiitapus, but an opportu- 

 nity for close investigation has not yet occurred. They are 

 strengthened by minute calcareous plates, as usual, but not suffi- 

 ciently so to diminish their flexibility. 



The oral circle is composed of ten pieces not joined by their 

 extremities, as in Synapta, but placed side by side. The five to 

 which the retractor muscles are attached are bifid anteriorly ; 

 the alternating five are broad at the base and tapering to the 

 apex, shaped like the teeth of some sharks. 



The stomach is distinctly marked, but not very muscular. 



The respiratory trees cannot be fully traced, from the want of 

 perfect examples, but they are apparently well developed. 



P. IcEvigatus has not yet been found south of Cape Cod. It 

 lives in deep water, and often affords food for fish, from whose 

 stomachs most of our specimens have been procured. Of course 

 we know little in regard to its habits. 



The species is readily distinguished by its small size and the 

 comparative smoothness of its scales. 



Mr. Desor laid before the Society a paper by Mr. Charles 

 Whittlesey, on the Equivalency of the Rocks between the 

 Conglomerate and the Cliff Limestone in Ohio, as related 

 to the formations of New York in the northeast, and of 

 Kentucky and Indiana in the southwest. The paper was 

 referred to the Publishing Committee. 



Mr. Stimpson made some observations on the identity of 

 Nucula navicularis and N. tkraciceformis. 



He had recently ascertained that Nucula navicularis of Cou- 

 thouy, and Nucula thracicEformis of Storer, are the young and 

 old of the same species. He had for some time suspected this, 

 judging from the similarity of form, and structure of the hinge, 

 and he had recently obtained specimens from which he had 

 formed a series, showing completely the passage of one into the 

 other, which he exhibited to the Society. Mr. Stimpson also 

 mentioned that he had never found specimens of ihraciczformis 

 without also finding its young, the navicularis, at the same time. 



The shell will come under the genus Yoldia, of Moller, which 

 is included in Leda, by Prof. Forbes. As Dr. Storer's descrip- 



