M A L A C O L O G Y. 



from their position and shape ; and their characters 

 being- external, it is not surprising that they are 

 so frequently and so advantageously employed in 

 malacology. 



A better guide still might be found 'with regard to its 

 importance in many essential points of consideration, 

 that is the armature of the mouth, either at its orifice 

 or in its interior, because that would immediately 

 enable the naturalist to pronounce, with considerable 

 certainty, the nature of the animal's habits of feeding 

 in general, but the difficulty of the examination has 

 doubtless deterred the most able zoologists from using 

 it more frequently as a law of arrangement in the 

 system. Another character, still more preferable 

 possibly with regard to its great importance, because 

 it accords extremely well with the form of the shell, 

 may be drawn from the position, the form, whether 

 symmetrical or not, and even the structure of the 

 organs of respiration. But here again, unfortunately, 

 it requires no small anatomical skill to employ this 

 character, though they are most frequently nearly 

 external, and the shell, where it exists, peculiarly 

 adapted to their protection. But the part of organ- 

 isation in molluscs, perhaps as important to consider 

 as almost every other, is the generative apparatus 

 composed of the two sexes, but separate in that struc- 

 ture of each, united both of them in one sex, or only 

 existing in females ; here again the examination is 

 entirely anatomical, and very difficult to apply to a 

 system of malacology. 



The form of the testaceous covering of molluscous 

 animals is certainly extremely important as an acces- 

 sory guide to the malacologist ; it must, therefore, be 

 well considered, and taken in its various grades of 

 useful information. The first consideration we would 

 point out is the number of pieces or parts which enter 

 into its formation, univalve, subbivalve or operculated, 

 bivalve, tubivalve, or multivalve. Secondly, its posi- 

 tion on the animal's back dorsal, as in all the 

 CEPHALOPODS dorsal and ventral, as in a small pro- 

 portion of the same order, and of the ACEPHALO- 

 PHORA or bilateral, as it is in all the LAMELLI- 

 BRANCHIATA. Thirdly, the indices it furnishes with 

 regard to the organs of respiration ; that is, the exist- 

 ence of a channel or tube at the anterior extremity of 

 the aperture in univalves, or of a gaping or separation, 

 more or less considerable, of the posterior extremity in 

 bivalves. Fourthly, the information it affords respect- 

 ing its connexion with the muscular system of the 

 animal, which constitutes what are termed the muscu- 

 lar impressions, simple in univalves, and only visible 

 in the Patella, Hal,iotis,fyc. ; more or less complex in 

 bivalves, and formed, as we have explained, in this 

 article, by one, two, or even several adductor muscles, 

 one or more retractor muscles of the foot, the mar- 

 ginal or parallel ligula, the mark of attachment of 

 the edges of the mantle, and backwards by the im- 

 pressions of respiratory tubes. Fifthly, the form, 

 whether symmetrical or not, w hich determines a simi- 

 larity or a variation in the parts of bivalves. Sixthly, 

 the form of the aperture in univalves, and the man- 

 ner in which each side of the columella or its vitreous 

 deposit contribute to form and modify it. Seventhly, 

 the ligament and hinge of the two portions of bi- 

 valves, that is the position and form of the li-amentous 

 fastening, and the position, number, and shape of the 

 so-called teeth of the hinge, observing particularly that 

 each true species of mollusc has its special and almost 

 constantly undeviating form of hinge and teeth, as in 



the Area NotE and the Glycimcrix, here figured, in 



Area Nose. 



which a different arrangement for example is visible. 

 Eighthly, a very important and strongly defined 

 character, now more extensively consulted from being 

 better known through the light Gray and other mo- 

 dern writers have thrown upon it, is the presence or 

 absence of an operculum, its form, and. most particu- 

 larly, its structure ; this furnishes, whenever it is 

 known to exist and can be examined, a clear, strong, 



Glycimeris. 



and unobjectionable proof of the class of molluscs, 

 which is external and undeviating in all its characters 

 of the same species. In the numbers of shells we 

 have examined we have never found any of those 

 anomalies existing in the operculum, or those singular 

 freaks of nature which other portions of molluscs 

 frequently exhibit ; their characters are simple and 

 uniform. It is constantly attached to the foot of the 

 animal possessing it, at the upper or lower side, clos- 

 ing the aperture of the shell completely, either at its 

 opening, or more or less internally distant from it. 

 Some authors Adanson of the number have con- 

 sidered it analogous to one of the valves of a shell, but 

 its position, with regard to the animal's body, decid- 

 edly contradicts that opinion : the two valves of a 

 bivalve are placed one on each side of the animal's 

 body, in almost every instance, while in the opercu- 

 lated molluscs, the shell alone, dependent on the 

 mantle, occupies its dorsal side constantly, and the 

 operculum never has any connexion but with the 

 upper or dorsal part of the animal's foot, sometimes 

 at the angle of its junction with the pedicle of the 

 body, rarely at its posterior extremity, and most fre- 

 quently midway. It is evidently the product of the 

 skin which covers the foot, and is, doubtless, an excre- 

 tion of horny or calcareous matter, but how a plain 

 surface of an oval or circular shape can produce a 

 matter whose molecules are spirally rolled or laid on 

 a flat surface, often with great regularity, sometimes 



