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 laminz, 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 eretaceous 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 effect 
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 
persevermg, 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 unfrequently 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. . 
