a fossil genus of Bivalve MoUusks. 347 



Example. 



Trichites nodosus. Plate X. 



(Great and Inferior Oolite.) 



Shell quadrately curved, with longitudinal waved nodose ribs j 

 ribs few, diverging, some bifid. The larger valve convex, the 

 lesser rather concave, with nodules nearly obsolete, disposed 

 in two concentric series. 



Our specimens differ much in the degree of convexity and cha- 

 racter of the ribs, the latter being occasionally scarcely distin- 

 guishable; the terminal umbones are very much curved and 

 turned forwards. The planking beds of the Great Oolite at Min- 

 chinhampton Common and freestone of the Inferior Oolite near 

 to the same locality produce it, but it has not been recognized in 

 the upper portion of the Inferior Oolite ; from the intractable 

 character of that rock, however, this circumstance must not be 

 considered as conclusive of its absence. Our examples in point 

 of size convey but a very inadequate notion of the magnitude 

 often attained by the genus ; but it would appear that the larger 

 sections belong to the second, or possibly even to a third and 

 more gigantic species. The Pinna Saussurei [Pinnegene of Deluc) 

 is distinct from our Cotteswold shells ; his figures, taken from 

 specimens broken and partly enveloped in the stone, do not con- 

 vey any precise idea of the external form, but the portion which 

 exhibits the character of the surface is altogether different. 



Trichites undatus. 

 (Inferior Oolite.) 



Shell oblong ; umbones . . . . ; ribs obscure, few, concentric, irre- 

 gular and undulated, sometimes obsolete in the larger valve. 

 The smaller valve unknown. 



Length 9 inches, breadth 7 J inches. 



The only well-preserved example in our possession has lost the 

 terminal extremity, but in other respects is nearly perfect ; the 

 hing&-line is more nearly horizontal than in the T. nodosus, and 

 the entire form is less convex ; the other valve, though attached, 

 cannot be cleared from the hard matrix. It occurs abundantly 

 in the upper ragstone of the Inferior Oolite, a rock which usually 

 defies all attempts to separate the shell in a tolerable condition ; in 

 this instance a large portion of the surface fortunately coincided 

 with the natural parting of the bed. 



