560 



SHELL. 



assigning to them a similar origin with the 

 preceding. We seem justified in doing so, 

 however, by two considerations. In the first 

 place, where the fully-formed shell is destitute 

 of cellular arrangement, this may be frequently 

 detected in the embryonic shell ; as the author 

 is informed by Dr. Leidy of Philadelphia, who 

 has carefully studied the embryology of many 

 Mollusca. And secondly, there are certain 

 shells which exhibit so complete and gradual 

 a transition from a distinct cellular arrangement 

 to an apparently homogenous structure, that 

 we can scarcely doubt the common origin of 

 both substances. This is particularly well 

 seen in the common Mya arenaria, a careful 

 examination of which shell brings to light nu- 

 merous interesting varieties of cellular organ- 

 isation. Thus in Jzg.413. we see in one part of 



Fig. 413. 







Section of shell of Mya arenaria, showing in one part 

 distinct cellular partitions, with large nuclear spots ; 

 whilst in another part of the same layer, the cell- 

 boundaries become fainter, and then disappear alto- 

 gether. Magnified 150 diameters. 



the section a very distinct set of cell-boun- 

 daries, with a large nuclear spot in the centre 

 of each cell ; whilst on the other side we ob- 

 serve that the cell-walls have completely dis- 

 appeared, the nuclear spots, however, still 

 remaining to mark the cellular origin of the 

 substance. A little further on, these also 

 might disappear, and thus all traces of the 

 original organisation might be lost, though no 

 reasonable doubt could be entertained as to its 

 prior existence. A very curious variety of 

 cell-structure is seen in the large hinge-tooth 

 of Mya, in which there is a layer of large cells 

 occupied by carbonate of lime disposed in a 

 radiated form of crystallisation, resembling 

 that of the mineral called Wavellite. Ap- 

 proaches to this beautiful arrangement may be 

 seen in many other shells. Here, too, we 

 find the partitions between the cells gradually 

 becoming less distinct, as we pass from this 

 peculiar stratum into the surrounding substance, 

 until we lose them altogether. In general, a cel- 

 lular layer may be detected upon the external 

 surface of bivalve shells, where this has been pro- 

 tected by a periostracum,or has been prevented 

 in any other mode from undergoing abrasion : 



thus it is found occasionally in Anemia and 

 Pecten, and generally in C/iama, Cleidothcerus, 



Fig. 414. 



Section of the hinge-tooth of Mya arenaria, showing 

 radiating arrangement of carbonate of lime within 

 the cells, and the gradual disappearance of the cell- 

 boundaries, so that the texture becomes homogeneous. 

 Magnified 80 diameters. 



Trigonia, Anatina, Solen, Glycimeris, Solemya, 

 &c. In the last-named genus it is very firm, 

 and leaves a distinct membranous residuum 

 after the calcareous matter has been removed 

 by acid, which is not the case with the others. 

 The cells of which the outer layer of the shell 

 is made up are frequently rather fusiform than 

 prismatic in their shape, and are disposed with 

 their long axes nearly parallel to its surface, 

 so that their extremities " crop out" very ob- 

 liquely on its exterior, where their rounded 

 terminations, containing nuclei, may often 

 be distinguished when the surface has not 

 suffered abrasion. (See Jig. 416.) . 



The internal layer of Bivalve shells rarely 

 presents a distinct cellular structure, when 

 examined in a thin section ; and the residuum 

 left after decalcification is usually a distinct but 

 structureless membrane, closely resembling 

 the "basement membrane" of Mr. Bowman. 

 (Mucous MEMBRANE.) This form of shell- 

 substance may consequently be distinguished 

 as membranous. In the MargaritacecB and many 

 other families, this internal layer has a na- 

 creous or iridescent lustre, which depends (as 

 Sir D. Brewster has shown*) upon the stria- 

 tion of its surface with a series of grooved 

 lines, which usually run nearly parallel to each 

 other. As these lines are not obliterated by 

 any amount of polishing, it is evident that 

 their presence depends upon something peculiar 

 in the texture of this substance, and not upon 

 any mere superficial arrangement. When a 

 piece of nacre is carefully examined, it be- 

 comes evident that the lines are produced by 

 the cropping-out of lamina? of shell situated 

 more or less obliquely to the plane of the sur- 

 face. The greater the dip of these laminae, 

 the closer will their edges be ; whilst the less 

 the angle which they make with the surface, 



* Phil. Trans. 1814. 



