|3flpuTnr JDictirjuan) nf Slmmatrtt jtature. 613 



The following observations, which we 



abridge from an article in ' Brande's Dic- 

 tionary of Science,' &c., are inten<led to sup- 

 ply the best additional information we can 

 ive on the subject, consistent with our con- 

 ined limits Shells, instead of consisting, 

 like bones, of living organized substance 

 permeated by blood-vessels, absorbents, and 

 nerves, are mere inorganic laminated, con- 

 cretionary, or crystalline deposits of calca- 

 reous earth, more or less combined with 



burrows inhabited by these Molluscs. But 



all the species in which the bivalve shell is 

 inadequate to the protection of the whole of 

 the body derive extrinsic defence by bur- 

 give on the subject, consistent with our con- rowing in sand, stone, or wood ; and they 

 fined limits Shells, instead of consisting, also commonly line their burrows with a 



layer of smooth and compact calcareous 

 matter, forming a tube. In all the Lamel- 

 libranchiate Bivalves which are free, the 

 two valves are symmetrical, and the shell is 

 termed cmiivalre ; in all those which adhere 

 Ibuminous matter : they are also formed i by one of their valves to foreign bodies, this 

 in the skin, and are appendages to the dermal valve is deeper and larger than the unat- 

 eystem, which in all cases of animals is the tached valve: such shells are termed ine- 

 principal seat of variety. In many cases, qttivalve. If the shell of the common Cockle 

 therefore, there exists very little correspond- \ (Cardium edu'.e) be examined, each valve 

 ence between the structure or even the pre- ! will be seen to be produced into a conical 

 sc-nce of a shell and the general character of 

 the organization of a mollusc ; and the ab- 



sence of uniformity between the condition 

 of the shell in closely allied species is exem- 

 plified in the highest as well as the lowest 

 cl:isM-i of the molluscous sub-kingdom 

 The formation of a shell commences with 

 the exudation of layers of albumen from the 

 outer surface of the mantle or skin of the 



prominence, bent towards, and nearly meet- 

 ing at, that part by which the valves are 

 joined together. These prominences are 

 termed the ttmbont*. The apex, or beak of 

 the umbo, corresponds to the apex of the 

 univalve shell, and is the point at which 

 the development of the bivalve commences. 

 When the apex if directed in the transvi -r>o 

 plane of the shell, and so placed that a bi- 



cmhryo mollusc, which is generally followed section of the shell in that plane through 

 by the admixture of rhombic or prismatic the apices shall divide the valve Into two 

 crystalline particles of the carbonate of lime : equal parts, the shell is termed equilateral : 

 and this first-formed shell of the embryo of this form the common scallop (Pecttn) is 

 constitutes the nucleus of the shell of the an example. When, upon a similar division, 

 mature mollusc. The nucleus id developed ' a slight difference is observed in the two 

 in most cases before the embryo quits the 

 egg-coverings ; but it is never coequal with 

 the first formation of the animal ; it is pre- 

 ceded by several distinct stages in the deve- 

 lopment of the embryo. The subsequent 



valves, the shell is termed sub-tquilateral ; 

 but where the difference is well marked, it 

 is an inequilateral bivalve. When the cir- 

 cumference or margin of one valve flts ex- 

 actly at every part to that of its fellow, it 

 is said to be " regular," or entire ; but if it 

 be notched at any part, so aa not to come 

 Into contact with the corresponding part of 

 the opposite valve, it is " irregular," or emar- 

 ginate. The most important part of the 



growth of the shell depends upon the depo- 

 sition of fresh layers to the inner suiface 

 of the circumference of those previously 

 formed ; beyond which the new-formed 

 layers extend in proportions which deter- 



mine the figure of the future shell In many i margin is that which is modified to form the 

 Univalves, the aperture of the shell is entire ; ' joint or hinge upon which the two valves 

 in others, it is broken by a notch, or per- I open and shut. This part is called the 

 forated by one or more holes j or a portion j "cardinal edge," and generally presents cer- 

 of it ia produced into a canal or siphon ; or j tain prominences and depressions, the pro- 

 it may present a pallia! notch opposite to ; jections of one valve interlocking with the 

 the siphon. These modifications are im- i depressions of the other. The projections 

 portant, on account of their relation to cer- I or u teeth," together with the cavities or 

 ns : " cardinal pits," are very regu 



tain conditions of the respiratory organs : 



very regular in their 



thus the conchologist, in grouping together : formation in each genus and species of bi- 

 all the spiral univalve shells of which a part I valve. What is of more importance is, that 

 of the margin was either notched or produced j every modification in the structure of the 

 into a grooved siphon, would really indicate i hinge is generally found to coincide with 

 a very natural tribe of Mollusca, every spe- | some recognizable and more or less important 

 cies of which he might be assured was aquatic i difference in the organization of the soft 

 and marine, and breathed by means of two 



frills having a pectinated structure, to which 

 the water is conducted by a fleshy tube. 



rts ; so that conchologists have justly at- 

 great value to the characters derivable 

 from the hinge, especially for the purpose of 

 When the teeth are 



Were a like correlation between the shell generic distinctions. 



and its inhabitant to hold good in other fa- ! situate beneath the apex or centre of the 



inilics of Mollusca, the classification of Shells hinge, they are called cardinal; when they 



would then be a subject of much importance, | are removed from the centre of the hinge, 



and worthy the attention of the scientific 



naturalist : uufbrtunately, the reverse of this 



is frequently the case. True bivalve Shells 



are peculiar to the Acephalous Mollusca i 



and their presence is constant, although they 



are in a few instances too small to cover the 



whole of the body, and in the ship-borers 



(Teredo) exist only as small instruments, 

 limited to the function of excavating the 



they are named lateral teeth; when two only 

 are present, one is called anterior, the other 

 posterior ; when there are three, they are 

 distinguished respectively as the anterior, 

 median, and posterior teeth ; but when the 

 hinge is composed of a great number of teeth, 

 it is said to be "serial," as in Area. The 

 direct medium of union of the two valves ia 

 a dense fasciculus of elastic albuminous 



3 a 



