ar4 PALAEONTOLOGY OF NEW-YORK. 



Treiiiatospira liirsuta. 



PLATE XLV. 



Jtrypa hirsuta : Hall, in Tenth Report on the State Cabinet, p. 108. 1857. 

 Trtmatotpira hirsuta : Id. Thirteenth Report on the State Cabinet, p. 101. 1860. 

 Jltkyru (?) ehloe : Billimosi Canadian Journal, p. 282. 1860. 



Shell depressed-orbicular in the young state ; becoming subtrilobate by 

 the gradual development of a mesial fold and sinus, and often gibbous 

 in the older specimens ; valves subequally convex ; hinge-line extend- 

 ing about two-thirds the width of the shell. 



Ventral valve usually a little more gibbous than the opposite ; greatest 

 convexity above the middle of the shell, whence it curves regularly to 

 the apex, which is terminated by a circular foramen, or more often 

 truncated below by the summit of the opposite valve : contour regu- 

 larly curving to the cardinal and lateral margins. The mesial sinus 

 becomes gradually developed above the middle in full grown shells, 

 and is very conspicuous towards the front, having the sides curving 

 and rarely strongly defined. The false area is not visible beneath the 

 beak. 



Dorsal valve regularly convex in young shells ; becoming elevated in 

 the centre, and a mesial fold gradually developing itself, till in old 

 shells it becomes very conspicuous towards the front. The sides are 

 pretty regularly convex, and curving towards the margins. 



Surface marked by from thirty to forty simple low rounded striae, which 

 are obscure towards the beaks, but become larger and more conspicuous 

 towards the margin : these are crossed by fine close concentric lines 

 of growth, and more distant imbricating lamella;. The surface ordi- 

 narily preserved is granulose ; but when perfect, it is covered by 

 minute seta or spinules, the bases of which, remaining, give the papil- 

 lose character. Entire shell structure punctate. 



The interior of the ventral valve shows two strong teeth, which are 

 extended in low plates along the sides of the rostral cavity to the mar- 

 gins of the muscular area, which is broad, flabelliform, and scarcely 

 defined on the front and lower lateral margins. 



