a fossil genus of Bivalve Mollusks. 347 
EXAMPLE. 
Trichites nodosus. Plate X. 
(Great and Inferior Oolite.) 
Shell quadrately curved, with longitudinal waved nodose ribs ; 
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 74 inches. 
The only well-preserved example in our possession has lost the 
terminal extremity, but in other respects is nearly perfect ; the 
hinge-line is more nearly horizontal than in the 7. 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. 
