MALACOLOGY. 



91 



two means are then in unison, and we may be certain 

 that the method is good. 1 " 



We will proceed to explain "shortly the system we 

 have now adopted as our guide in malacology, and it 

 will be necessary to point out some general prin- 

 ciples to assist in discovering the distinctions of spe- 

 cies, the most difficult part of all the branches 

 of natural science, but most particularly in this 

 above the others on account of the shell hav- 

 ing so long been considered the only guide for 

 the distinction of species ; and as 

 before remarked, the indications 

 given by the structure of the shell 

 must supply the information we 

 want, where the anatomy of the 

 animal is totally unknown, as for 

 instance the Aspefgittum, here 

 figured, is classed as following its 

 congeners, the Gaslroc/ia-na, Cla- 

 vagel/a, and others of the family 

 Pylondea, from characters common 

 to all of them ; though it is diffi- 

 cult to assign any reason or use 

 for the perforated holes in the disc 

 of its lower end, without they are 

 to give a passage to the fibrous 

 muscles of the animal's foot, in the 

 character of a byssus, by means 

 of which the animal affixes itself 

 to submarine bodies, in a situation 

 more or less vertical, the smaller 

 extremity of its tube above, and 

 the head of the animal down- 

 wards ; this is at present merely 

 conjectural, the animal not being 

 known. The species of any one 

 series of animals cannot correctly be determined 

 without taking into consideration .the anatomical 

 structure of the apparatus of generation, and par- 

 ticularly its accessory parts, although a strict coin- 

 cidence may exist or not in the other parts of organic 

 structure. These latter are not always so essential 

 from the dubious circumstance that they are frequently 

 only observable in such parts as cannot be called organs 

 possessing an use, but no functions such as shells. This 

 principle may be rigorously applied to mollusca, 

 since all the true species that physiologists have exa- 

 mined in a very large and numerous genus, the 

 Helices, for example, there has always existed some 

 important difference in the generative organs of 

 course much more marked in the separate sexes, as 

 is evident also in the Murcx, Buccinum, &c. ; those 

 distinctions are additionally defined by the size of 

 the shell, which, in the females, is always of a greater 

 capacity than that of the males. 



Adanson, who has devoted much time to the study 

 of molluscous animals, assures us that even other 

 parts of the animal, such as the lobes, and the appen- 

 dages of the mantle, vary considerably with age, the 

 period of the year, and consequently the shell also, 

 whose differences are not confined to the colour, its 

 being smooth or rugged ; its thickness, its size, the 

 development of its various bands or tubercles ; but 

 that they extend likewise to the form of the opening, 

 and the proportion of other parts. If, therefore, the 

 sex or age present an evident influence in determining 

 the distinctions of molluscous animals, and conse- 

 quently their shells, it i.s not less evident that in appre- 

 ciable circumstances may influence, in a smaller degree, 



Aspergillum. 



the animal ; and in a larger degree, the shell, whose 

 colour, proportions of size, and different parts, vary 

 remarkably in different examples of the same species. 

 This may be observed in the Helix ncmoralis, Lymcea 

 stagnalis, and others we have an easy opportunity of 

 examining in confirmation of this fact. By a parity 

 of reasoning, we may readily imagine, that a variety 

 of circumstances, to a certain point 

 appreciable, such as climate and lo- 

 cality, have decidedly influenced 

 for a series of time, and almost as it 

 were upon fixed principles, the suc- 

 cession of individuals of the same 

 species, in their size, colour, &c., and 

 even in the form of their opening, 

 and the peculiar characters of their 

 superficies, compared with others 

 precisely of the same genus, living 

 for centuries in an opposite locality. 

 This may be termed the geogra- 

 phical distribution of species, very 

 marked certainly in many instances, 

 but, in truth, not really affecting a 

 scientific arrangement ; for it inva- 

 riably happens, that when these 

 varieties are assembled, we find them 

 range insensibly with each other, and 

 thus form but one family. This is 

 exhibited in the Ceritheum, whose 

 species differ so greatly in their ex- 

 ternal form, but all possess the same 

 generic characters of structure. A 

 most interesting but difficult scheme 

 might be formed of these insensible 

 gradations in every reign of natural 

 history, proving indubitably the unity 

 of general purpose, exhibited in 

 them all, according to their peculiar 

 duties and stations in the scale of 

 animated nature. 



A minute examination of living molluscs is 

 also indispensable to the geologist, as it mainly 

 guides his conclusions in furnishing a good na- 

 tural guide in resolving the problem of the analo- 

 gous strata of secondary and tertiary formations. 

 The connexion between molluscs and the geological 

 structure of the earth is extremely manifest and 

 highly important, for without seeking to ascertain 

 here the difficult fact, whether shells have borrowed 

 from inorganic matter their bases, or, as we think, con- 

 tributed to effect the marvellous changes produced on 

 the surface of the globe b\ their decomposition and 

 masses in various stages of existence ; one thing is 

 quite clear, that the manner in which these deposits are 

 made, differs essentially in various places, and under 

 various circumstances, according principally to the 

 habitat of molluscs ; whether beneath the sand, or 

 mud, or free in the bosom of the deep ; thus, the 

 oyster in all countries forms beds and banks of various 

 extent and thickness, horizontally placed, in which 

 at this moment they remain precisely similar to the 

 position they occupied in former ages, and almost 

 without any other admixture of different bodies ; this 

 fact is less evident in many other of the mollusca ; but 

 all the genera of molluscs living in the sand or mud, 

 vertically placed, must of necesMty form beds of their 

 spoils, since the increase of number is made by a deposit 

 of the newly-born above the parents, who in course 

 of time become buried by their own offspring, and, 



Ceritl cum. 



