ATRYPA OF THE HAMILTON GROUP, &c. 
S3S 
spines, which, when worn off, leave the ordinary lamellose surface. The 
spaces between these projecting lamellae are marked by fine thread-like 
striae. 
In the separated valves, the hinge-line is often nearly straight, the mus¬ 
cular area of the ventral valve is short and broad, the length from the apex 
being about equal to the width. There is a slight thickening of the shell 
at the base of the rostral cavity. The surface around the muscular area is 
papillose, and limited by a thickened border, except in front, where 
it is discontinued. Fine vascular markings are sometimes visible near the 
margin. In the dorsal valve there is a thickened septum in the upper part 
of the muscular area. 
The spires of full grown individuals about fifteen turns in each. 
In casts of this species from the Hamilton group of Maryland and Vir¬ 
ginia, we find the same characteristic features preserved as above described. 
I have adopted the name Atrypa spinosa , although I think this may be identi¬ 
cal with the species known as Atrypa asptra or Atrypa reticularis , var. aspera, of 
European authors ; but I wish to preserve for our shell a specific designation. 
Prof. M‘Coy, Mr. Davidson and other European authors adopt the opinion that 
the variety A. aspera passes “ by insensible gradations” into well-marked A. 
reticularis. I must insist, however, on a different status for our shell. Passing by 
the specimens from the Corniferous limestone, where the shells are exfoliated and 
changed by silicification (rendering discrimination not always so easy), we have 
only to examine large collections from the Hamilton group to discover a constant 
and well-marked distinction. I have collected and examined many hundreds of 
specimens of Atrypa reticularis , from various localities of the Hamilton shales in 
New York, and a smaller number of Atrypa spinosa. The former is much the 
more abundant, and occurs in locality where the latter is not found, though fre¬ 
quently both occur in the same localities, and never for one moment will the 
collector hesitate as to their distinct specific distinction. 
The full-grown shells of A. spinosa are proportionally broader and more robust, 
and from about twelve to twenty-five or twenty-six ribs may be counted on the 
Surface of each valve, according to the size of the shell and the degree of devel¬ 
opment towards the margin. In specimens,of A. reticularis of similar size, more 
than twice as many costse can be counted, while they do not become obsolete on the 
