TREMATOSPIRA. OP THE HAMILTON GROUP. 275 



The interior of the dorsal valve shows a strong deeply bilobed cardi- 

 nal process, with the bases of slender crura ; the teeth sockets are large 

 and deep ; there is a low median crest or septum, Avhich is somewhat 

 Btrong above, but dies out about the middle of the shell. In specimens 

 which have been cut to show the spires, these appendages are slender, 

 with about ten or eleven turns on each side. 



The proportions of length and breadth are about as three to four. 

 The largest specimen observed is a'little more than three-fourths of an 

 inch in length, by an inch and one-sixteenth in width ; while many of 

 the specimens are less than half these dimensions. A well-formed speci- 

 men of about three-fourths of an inch in length by one inch, has a depth 

 of nine-sixteenths of an inch. A very gibbous specimen measures three- 

 eighths of an inch in length, nine-sixteenths in width, and half an inch 

 in depth. 



The general aspect of this shell is similar to T. muUistriata; but the form is 

 usually less gibbous, the sinus more distinctly defined, and the striae are 

 esscntiallj' simple. It is distinguished externally from Atrypa, by the granulose 

 surface and punctate character of the shell. 



This fossil is not abundant, though not exceedingly rare. It is principally 

 restricted to the western part of New-York, and has been found in Canada West. 



Tbexatospiba niBSUTA (Hamilton group). 



'i^P 



^^^ 



10 



Fios. 7 & 8. Dorsal and front, views of a specimen. 

 Fios. 9 & 10. Interior of tlie dorsal and ventral valves. 



Geological formation and localities. This species occurs on the shores of 

 Canandaigua lake and at Bloomficld, Ontario county ; at York and Moscow, Li- 

 vingston county ; at Paviliou and Darien, Genesee county, and at Eighteen-mile 

 creek in Erie county, New- York. In Canada West it occurs near Widdcr station 

 on the Grand Trunk railw.iy. 



I have received this species from Dr. James Knapp, of Louisville, collected from 

 the coruiferous limestone at the Falls of the Ohio. 



The following figures in wood are illustrations of some of the typical species of 

 TREM.\To.sprRA, froiu the Lower Helderberg group, described, and heretofore 

 given in the State Cal)inet Report. 



