84 MOLLUSCA FROM THE GREAT OOLITE. 



it is obtained, and a little search discloses the crypts in such extraordinary numbers, as to 

 indicate that the Lithophagidce then existed in a force which would not have been expected 

 from the small number of instances in which their tests are preserved. In the shelly beds 

 of the Great Oolite, the convex valve of Trichites is usually covered, and even loaded with 

 small adherent oysters; but through these masses the perforations of the Lithophagida are 

 found to have passed, a sufficient proof that the operations of the latter mollusks com- 

 menced posterior to the occupation of the adherent shells, and leading to an inference 

 that the latter may have pertained to the living examples of Trichites. Our Great Oolite 

 examples of the genus convey but a very inadequate idea of the magnitude which it 

 sometimes attained ; the upper division of the Inferior Oolite has disclosed sections of the 

 valves upwards of two feet in length, and two inches in thickness. A shell imbued with 

 such peculiar fragility, must have been unfitted to exist upon the bed of a littoral deposit 

 exposed to the attritions and accidents to which such a position must have been incident, 

 but in which, nevertheless, we find their remains ; it seems more probable that they lived 

 like the Myadae, buried and defended in mud or sand, and that it was only by the 

 denuding action of currents that their shells became exposed, and rolled with other 

 fragmentary bodies. 



In seeking for the generic forms allied to Trichites, we are reminded of the Catilli ; 

 the structure of the shell is alike in both genera, and the general figure is not very dissimilar, 

 but the regularity of one contrasts with the irregularity of the other ; the character of the 

 surface more especially is distinct, the regular concentric folds of the Catilli have no 

 affinity with the nodose and laminated surface of Trichites, nor can the recurved solid 

 umbones and thick crenulated hinge plate of the former genus find any similitude with 

 the Pinna like termination of Trichites. But if the character of the apex be allowed to 

 resemble that portion of Pinna, we may search in vain for any other point of affinity 

 with that genus; the structure of the shell in each differs materally; in Pinna it consists 

 of two distinct layers, the external one of which is fibrous, but the internal is that of 

 ordinary shell or nacreous, a structure tending to obviate the fragility which pertains to 

 the fibrous structure of the outer layer, and very much resembling a method practised in 

 the mechanical arts, for giving increased strength to thin layers, in substances whose 

 fibres pass in different directions ; by this contrivance, a shell very thin is made to possess 

 a considerable degree of elasticity and strength; the other particulars, in which Pinna 

 differs from Trichites, embrace every remaining generic character, as the equivalve form, 

 its regularity, the gaping truncated posterior extremity, and lastly, the muscular impres- 

 sions, of which Pinna has two. The preceding comparison with Pinna has been made in 

 consequence of several authors, who confessedly had acquired only a very imperfect 

 knowledge of Trichites, having classed the Pinniyenc of Deluc, with Pinna, under the 

 name of Pinna Saussurei. 



It is now known that Trichites is abundant in the Oolitic rocks of England, and is 

 found over extensive areas, but it is not confined to one of the Oolitic formations merely, as 



