BIVALVIA. 33 



character of the surface, so that species are not easily discriminated j the costse, nodules, or 

 other elevations which are occasionally present upon the surface, become indistinct or 

 even vanish altogether; but their broad flexure upon the posterior side and their irregidar 

 sinuous hinge border are invariably conspicuous. 



The structui'e of the test consists of closely -packed perpendicular fibres of a columnar 

 aspect, which are traversed transversely by calcareous laminae of extreme tenuity, parallel 

 to each other, and sometimes of a colour difierent from the rest of the shell ; they occur 

 at very uncertain distances, appearing in the sections as so many fine lines, these thin 

 laminae give both to the external and internal surfaces of the valves a perfectly smooth 

 appearance, and in some sections a dozen or more of them may be counted, they indicate 

 successive additions of thickness to the test during the growth of the animal. The 

 muscular impression exhibits the mode of growth in a very clear manner, the necessary 

 addition of perpendicular fibres around its circumference producing a sudden elevation, or 

 step like surface at the border of every successive zone of increase. The position of the 

 muscular impression is rather posterior to the centre of the valves, or nearer to the posterior 

 and superior border ; there may also usually be observed upon the inner surface of the 

 flat valve, at a little distance from the anterior border and parallel to it an elongated 

 swelling, or rounded prominence, having exactly the contour of the outer border, and 

 exhibiting the appearance of having formed the outer border at a former stage of growth, 

 a feature precisely similar to that which is observed in certain oysters. The irregular 

 swellings upon the surfaces of the valves do not coincide with the surface of the interior, 

 sections of the thicker specimens often exhibit this circumstance in a very striking manner, 

 and likewise a general irregularity and inequahty in the thickness of the test ; the inner 

 surfaces of the valves, though smooth, are singularly uneven, and it is not uncommon to 

 observe an occasional thickness in the test of seven or eight lines, terminate towards the 

 posterior border in a considerable degree of tenuity and delicacy. It would seem that the 

 transverse laminae, whatever may have been their original structure, impeded fracture 

 only to a very limited extent, for we find that in most cases the fracture is directly across 

 all the laminae, occasionally indeed the fracture has been arrested at the surface of one of 

 the laminae, and has followed the plane of its surface for some distance, an indication that 

 its structure was lamellar. The fibrous structure then was very fragile, in the fossil state, 

 fracture in the direction of the fibres takes place upon any slight concussion, however thick 

 may be the test; and M'ith the living shells the same circumstance seems to have obtained, 

 for in the majority of instances, Trichites acquired its fossil state in the condition of fragments 

 only, and these occur in such numbers, both in the Great and Inferior Oolite, as to 

 indicate that this genus occupied a very prominent position amongst the marine fauna 

 of the lower Oolitic epoch. The valves of Trichites (more especially the older and thicker 

 specimens), are perforated, and sometimes literally honeycombed with little crypts of 

 Lithophagida, in which, occasionally, the valves of the shells may be discovered ; these 

 perforations are a constant feature pertaining to Trichites, from whatever formation or bed 



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