RHYNCHOSPIRA OF THE HAMILTON GROUP.' 377 



Ventral valve broadly suboval, regularly convex, or with a scarcely 

 perceptible mesial depression. The beak is much elevated above the 

 opposite valve, gently arcuate, with the apex perforated, and the 

 space between it and the summit of the dorsal valve closed by two 

 convex deltidial plates which are excavated on their inner and upper 

 margins, forming an elongate oval perforation communicating above 

 with the opening of the beak. 

 Dorsal valve often nearly as wide as long, moderately convex, with the 

 greatest convexity a little above the centre. There is a slight depres- 

 sion along the middle in some individuals, but this is an obscure or 

 inconstant feature. 

 Surface marked by about twenty-four (varying from 20 to 25) simple 

 regular plications, which* are gradually enlarged towards the margins 

 of the valve. On the dorsal valve, two or three of the central plica- 

 tions are depressed, causing a slight sinus in front; but there is no 

 corresponding elevation on the opposite valve. The shell structure is 

 punctate throughout. 

 The interior structure is not known. 



I Lave referred this species to Rhynchospira, from its generally similar aspect 

 to those of the Lower Helderberg group ; but the large deltidial plates and double 

 perforation have not been observed in those species ; though in T. rectirostra, 

 there are deltidial plates closing the space below the apipial foramen. It is still 

 desirable to examine specimens for the presence of the spires or crura, as these are 

 unknown. 



The prevailing form of the specimens is that described ; but rarely there occurs 

 one that is more elongated. The species has the general aspect of Rhynchonella; 

 but it ii readily distinguished by the narrow deprcision of two plications on the 

 dorsal valve, and by the punctate structure of the shell, as well as by the forameJi 

 and deltidial plates when these features are preserved or exposed. 



Geological fotination and localities. In the shales of the Hamilton group, on 

 the shore of Canandaigua lake, and near Bellona, in Ontario county, New- York. 



RHYNCHOSPIRA NOBILIS. 



Rhynchospira nobilis : Hall, Thirteenth Report on the State Cabinet, p. 83. 1800. 



A re-examination of the specimens has left some doubt regarding the generic 

 relations of this fossil, and the description is deferred to another part of the 

 volume. 



