712 



CONCHIFERA. 



guished into anterior and posterior, and they 

 are frequently unequal. 



If we now turn to the particulars of the 

 external surface of the shell of the conch i- 

 fera, we shall find many points worthy of 

 being attentively noted. In a very great 

 number this surface is covered with a thin 

 and frequently deciduous lamina of a sub- 

 corneous and often filamentous substance, to 

 which the title of epidermis has been given. 

 This matter is secreted by the most external 

 edge of the mantle, but observers have not 

 yet stated in what manner the secretion takes 

 place, and what means the creature employs 

 to make this epidermis adhere so strongly to 

 its shell. The epidermis often occurs both of 

 considerable thickness and extent (Glycimeris, 

 Solemya), and thus constitutes an important 

 portion of the shell. In other genera the epi- 

 dermis appears to be wanting entirely, and in 

 others bears some resemblance to velvet of 

 thicker or thinner pile, and then consists of a 

 large quantity of short hair, standing erect, 

 and more or less closely set. In some species 

 these hairs become more scanty, but increase 

 greatly in length, as we perceive in certain 

 Archidae and Bucarides. When it occurs in 

 certain species the successive growths of which 

 are manifested by irregular ridges, the epider- 

 mis is irregularly squamous. The epidermis 

 is insufficient to furnish any generic character 

 that can be depended on ; for there are certain 

 extremely natural genera in which some species 

 are covered with it whilst others are entirely 

 naked. 



The other particulars of the external surface 

 of the shell are soon glanced at : they consist 

 of stria, ridges or ribs, and furrows, which, 

 according to their direction, are distinguished 

 into longitudinal when from the hook they run 

 towards the inferior margin, and transverse 

 when they foliow an opposite course, that is to 

 say, when they run from before backwards ; 

 they are oblique, again, when they follow a line 

 in any way inclined to the longitudinal or the 

 transverse. These stria, ridges, and furrows, 

 may cross one another, and the shell is then 

 trellised. They may also severally present a 

 great variety of particular appearances, the de- 

 finitions of which may be found in the ordinary 

 elementary works on Conchology , but which may 

 all be learned much more rapidly from even a 

 very moderately attentive study of the shells 

 themselves than from any written description, 

 however minute and accurate. 



Internal surface. The inner surface of bi- 

 valve shells is commonly smooth and polished, 

 and often presents different colours which de~ 

 pend on the secretion of that part of the man- 

 t'e which produces the solid laminse of the 

 inner surface. The greater number of shells 

 affe white within, and many of them are na- 

 creous or like mother-of-pearl. Mother-of- 

 pearl would appear to be the consequence 

 of a molecular arrangement of the calcareous 

 matter intimately united in a constant ratio 

 with the animal matter by the combination 

 of which the shell is formed. The pro- 

 portion of the two substances does not ap- 



pear to be the same in the non-nacreous and 

 the nacreous shells ; there are some which 

 afford a much larger proportion of calcareous, 

 and others which yield a much larger propor- 

 tion of animal matter when analysed than is 

 usual. Naturalists are now generally aware 

 of the experiments, an account of which is to 

 be found in the Philosophical Transactions, 

 from which it appears that the nacreous lustre 

 is owing to the decomposition of light by an 

 infinity of asperities of excessive minuteness 

 which beset the surface of the shell. It has, 

 indeed, been found possible by means of an 

 impression from a mother-of-pearl surface taken 

 in sealing-wax especially, to transfer the power 

 of exhibiting corresponding phenomena to the 

 surface of the wax.* 



There is a variety of characters exhibited 

 by the interior of the valves which it is of con- 

 sequence to be familiar with. In shells which 

 have belonged to dimyary mollusks, two 

 muscular impressions of variable depth are 

 constantly to be found in the interior. Some- 

 times they are so superficial that they escape 

 an examination which might even be charac- 

 terized as minute. One of these impressions 

 is on the anterior side of the shell, the other 

 on the posterior. They are generally sub- 

 rotund ; sometimes, however, they are elon- 

 gated, which serves as an announcement that 

 the muscles were flattened. In some genera 

 these muscular impressions are of a particular 

 form, as may be observed in the Lucina for 

 example. It is a circumstance worthy of ob- 

 servation that the muscles of the animal shift 

 their place and come forward in the shell in 

 proportion as it grows, and it might have been 

 concluded, a priori, that this could not be 

 otherwise, when the mode of increment pecu- 

 liar to the class is taken into consideration. 

 On escaping from the ovum, a conchiferous 

 mollusk is already provided with its shell, of 

 course of very small size, and its two adductor 

 muscles ; and the relations of these muscles to 

 the shell and the other internal organs are the 

 same as at every subsequent period. When 

 the animal has attained to some lines in length, 

 and by the lapse of time to much larger di- 

 mensions, did not the muscles undergo a 

 gradual displacement the shell would be found 

 as thin at the summit as it was on escaping 

 from the egg, and the muscles prolonged into 

 the interior of the hook. Now, not only does 

 the shell go on increasing in thickness and the 

 hooks fill up, but observation shows that the 

 adductor muscles always preserve the same 

 relations and the same proportions. To study 

 in the best possible manner the successive dis- 

 placements of the muscular impressions, the 

 best mode is to saw a fossil oyster-shell length- 

 wise in a line passing from the summit through 

 the centre of the muscular impression. The 

 impression will then be seen beginning towards 



* [We have heard this point disputed. The 

 power which the sealing-wax had certainly gained 

 in some instances of exhibiting the mother-of- 

 pearl lustre was afterwards shown to depend ori the 

 wax having detached a minute film from the stir 

 face upon which it had been pressed. ED.] 



