RHYNCHOSPIRA OF THE HAMILTON GROUP. 
277 
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. 
Lhave referred this species to Rhynchospira, from its generally similar aspect 
to those of the Lower Helderberggroup ; 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 apicial foramen. It is still 
desirable to examine specimens for the .presence of the spires or crura^hs 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 is readily distinguished by the narrow depression of two plications on the 
dorsal valve, and by the. punctate structure of the shell, as well as by the foramen 
and deltidial plates when these features are preserved or exposed. 
Geological formation 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. 1860. 
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. 
