ON THE MICROSCOPIC STRUCTURE OP SHELLS. 19 



of the shells ; for Placuna and Anomia agree in several particulars, in which 

 both differ from the Ostracea. The principal part of the shell of the Pla- 

 cunidce consists of true nacre, the laminaB of which are peculiarly separable 

 from each otlier, thus in some degree corresponding with Terebratula and 

 other Brachiopoda. In the Oyster, the shelly layers are more divisible than 

 they are in most other Conchifera, and so far it approaches the Placunidce ; 

 but this divisibility is not nearly so great as in the latter. In the form of the 

 nacreous lineation, too, the Placunidce show more resemblance to Producta 

 than they do to the ordinary Conchifera. Their chief point of distinction 

 from the OslracecB is the entire absence of the prismatic cellular structure 

 which characterizes the latter, and the presence, in its stead, of a tubular 

 structure which is found in the nacre itself of Placuna and Anomia, but 

 more particularly in the yellowish external coat of the upper valve in the 

 latter genus (figs. 4-0, 41). The tubuli are about 1 -2000th of an inch in dia- 

 meter; they sometimes form a network parallel to the laminae, and sometimes 

 dip down and penetrate them obliquely or vertically ; the wavy direction of 

 the tubes is particularly evident in these shells. By these characters I should 

 have no difficulty in identifying a small fragment of a shell belonging to this 

 family, as I know no other shells which have so regular a distribution of 

 large tubes in their nacreous layers. 



XI. OstracecB. 



49. This family now contains only the genera Ostrea and GryphcBa, between 

 which there is a very close resemblance in general characters, so that it is 

 doubted by many conchologists whether they are really distinct, the one 

 passing gradually into the other. This correspondence exists also in their 

 microscopic structure ; in both we find a layer of prismatic cellular sub- 

 stance, in which the cells are very obliquely arranged, forming the margin 

 of each lamina (fig. 44), whilst the general structure of the shell is sub- 

 nacreous (§ 29). Between the recent Gryphcea and Ostrea, I have not been 

 able to detect any difference ; but in the Gryphcea incurva of the lias, I 

 have found the nacre perforated by scattered tubes, of which no trace exists 

 in Ostrea edulis. 



Xll. PectinidcB. 



50. In the several genera of this family, the structure of the shell is almost 

 exclusively membranous. There are generally two very distinct layers, an inner 

 and outer ; but there is no essential difference in their structure, the chief point 

 of distinction being usually in their colour, as in Pecten and Spondylus. I have 

 occasionally met with traces of cellular structure, especially on the external 

 surface of the shell ; but I am not inclined to believe that these are to be 

 regarded as constant, or as peculiarly characteristic of the group (fig. 42). 

 No distinct cellular layer can be obtained by the decalcification of the shell ; 

 but cells are seen here and there scattered among the folds of the basement- 

 membrane. Hence I am inclined to regard them simply as the remains of 

 the original calcigerous cells, by which the shell was at first formed. — The 

 most characteristic feature of the shells of the PectinidcB is the coarsely- 

 corrugated structure which they exhibit, both in their inner and outer layers 

 (fig. 43) : there is also, in some instances, an extremely delicate corrugation, 

 visible only with a high power, and giving to the shell the appearance of 

 possessing a delicate fibrous texture. Both these arrangements are seen in 

 tiie decalcified membrane, as in the shell itself. In some shells of this family 

 there is a very remarkable amount of tubidar structure ; in fact, I have no- 

 where found a more characteristic example of it than in Lima scabra, but it 

 is not constantly present even in species of the same genus. 



51. We shall hereafter find that this corrugated structure, with a greater or 



c2 



