346 Mr. J. Lycett on Trichites, 



the thin and delicate Pinna is produced by a double structure, 

 by its substance consisting of two layers, the outer being fibrous, 

 the fibres placed perpendicular to the surface as in Trichites; 

 but the inner one is nacreous or lamellar, a contrivance which 

 effectually obviates the fragility which pertains to the fibrous 

 structure. Trichites on the contrary has one structure through- 

 out ; the perpendicular fibres are crossed by a few extremely fine 

 parallel laminse, which do not break off the continuity of the 

 fibres, and impede fracture only to a very limited extent. The 

 genus Catillus, found only in the cretaceous rocks, is that which 

 seems to approximate most nearly to the present form. Both have 

 very much the same general figure, fibrous structure and thick 

 substance ; on the other hand, the Catilli are nearly equivalve and 

 regular, the hinge consists internally of a linear series of crenu- 

 lations, and the terminal character of the umbones is likewise 

 distinct. 



The position of Trichites in the conchological series should 

 therefore be near to Catillus; its irregularity is such that no 

 two of our specimens are exactly alike either in outline, con- 

 vexity or surface. Two individuals were nearly covered with small 

 adherent shells, a sufficient indication that their habits were se- 

 dentary or sluggish. The great fragility is certainly not the efiect 

 of fossilization, but a consequence of its structure, and must 

 always have existed ; the condition in which the shells are found 

 presents a sufficient confirmation of this idea. After frequent and 

 persevering, but for the most part fruitless exertions with the 

 hammer and chisel, we are enabled to state that Trichites usually 

 occurs in the state of distinct portions or fragments, and that 

 entire individuals are rare ; the fracture too not Cinfrequently is 

 found across the thickest part of the test. A mollusk, whose 

 shell was thus constituted, could scarcely have been the denizen 

 of a shelly beach within the influence of the tide, or exposed to 

 frequent rolling and collisions with other fragmentary bodies ; 

 its habitat must have been tranquil, and probably covered deeply 

 and defended by soft mud or sand. Specimens which occur in 

 the chaotic shelly beds of the Great Oolite, locally termed ' plank- 

 ing,^ may be regarded as travelled, and, as might be anticipated, 

 are usually in portions only, and these seldom large; individuals 

 perfect or approaching to perfect are the exceptions to the rule. 

 In the upper portion of the Inferior Oolite the conditions of sea- 

 bottom appear to have been somewhat different ; there is an ab- 

 sence of shelly detritus ; the valves of the Conchifera are most 

 frequently in apposition, and Trichites, as far as can be ascer- 

 tained, appears more frequently to approach to the perfect con- 

 dition. 



