714 



CONCHIFERA. 



produced, as we have already said, by the pro- 

 per retractor muscle of the siphons. 



Besides the muscular impressions of which 

 we have now spoken, several others of much 

 less importance have been partieadarized in the 

 greater number of the conchiferous mollusks. 

 All the species that have a foot have peculiar 

 muscles to move this organ, and these have 

 their fixed point of action on some point of 

 the interior of the shell. They are generally 

 divided into two principal fasciculi ; the one 

 runs to be inserted within the hooks, the other 

 in the Dimyaria proceeds to be attached before 

 and above the posterior adductor muscular 

 impression. In the Monomyaria, the foot of 

 which is generally rudimentary and without . 

 use, we observe nothing more on each side 

 of the body than a single small fibrous fas- 

 ciculus, the impression of which is found on 

 the inside of the hooks. Jn some genera of 

 Dimyaria, ar.d particularly in the Unio, we 

 observe three and sometimes four muscular im- 

 pressions belonging obviously to the adductor 

 muscles of the valves, which are occasioned 

 by the anterior adductor muscle in particular 

 being divided into two fasciculi, often of un- 

 equal size, as in certain Uniones, and some- 

 times equal and of considerable magnitude, as 

 among the Inclines. 



From the summary and concise view we 

 have taken of the principal facts in the organi- 

 zation of the Conchifera, very important con- 

 clusions, may be drawn with reference to the 

 classification of these animals. 



Taking the Conchifera, properly so called, 

 and looking narrowly into that which is of 

 most importance in their organization the ner- 

 vous system, we find two principal modifica- 

 tions, coinciding in a very remarkable manner 

 with the number of the muscles. This num- 

 ber of the muscles, permanently proclaimed 

 by the impressions they leave on the shell, 

 presents an important character by means of 

 which, while we define their limits somewhat 

 more strictly, we feel authorized in retaining the 

 two grand orders of Lamarck, the Conchi- 

 fera Dimyaria, and the Conchifera Mono- 

 myaria. A fact of some importance, and 

 brought to light by the observations of Poli, 

 is that a small nervous ganglion exists at the 

 point of commissure in those acephalous mol- 

 lusks which have the lobes of the mantle con- 

 joined. This peculiarity gives new conse- 

 quence to the characters drawn from the con- 

 joined or disunited state of the lobes of the 

 mantle. Unfortunately the circumstance is 

 not always indicated upon the shell ; it is, in 

 fact, only obvious upon those inhabited by 

 siphoniferous animals; it is quite inapprecia- 

 ble tipon those the inhabitants of which have 

 siphons so short as not to require a particular 

 retractor muscle to draw them within cover of 

 the shell. With regard to the other organic 

 characters which furnish data available in clas- 

 sifying the Conchiferous mollusks, these are all 

 of so little permanency that they are only 

 useful in supplying secondary hints for the 

 arrangement of families and genera. Thus 

 neither the branchiae nor the heart present any 

 chaiitctcr susceptible of generalization or of 



contrast. Better data might perhaps be ob- 

 tained from the conformation of the organs of 

 digestion ; but these organs have hitherto been 

 examined in comparatively so small a number 

 of genera and species that they cannot be 

 brought forward usefully in supplying cha- 

 racters for a general classification. If, as we 

 ourselves feel inclined to do, the hinge be 

 taken as the point of starting in the Pholades, 

 this part may be made the means of giving 

 excellent characters in its principal modifica- 

 tions for the establishment of genera. It is, 

 indeed, very remarkable that we should find 

 the characters as indicated by the hinge 

 almost constantly in harmony with those af- 

 forded by the rest of the organization ; and 

 with a few exceptions, relative to several ex- 

 tremely natural families, that of the Unios for 

 example, all that is valuable in the generic cha- 

 racters generally may be preserved along with 

 the characters supplied by the hinge. Ano- 

 ther character which may be usefully employed 

 in classification is assumed from the regularity 

 or irregularity of the shell of the animal ; in 

 generalizing upon this, like groups are obtained 

 in the two principal divisions of the Conchifera, 

 and the two principal divisions of the classi- 

 fication are referred to the simplicity or exact- 

 ness of the dichotomy, whilst natural groups 

 are preserved as much as may be in the linear 



arrangement. 



Method, it must ever be remembered, is an 

 artificial means of introducing order among a 

 series of observed facts, and of approximating, 

 according to the analogy of their organization, 

 the beings which nature has scattered over the 

 face of the earth ; method is a human creation 

 altogether, and in this light must it be viewed. 

 To be all it ought, every known fact must be 

 included, and the greatest possible amount of 

 organic relationships between the individuals 

 of each great class must be indicated. In an 

 exposition of facts seriatim, and as they occur 

 in a book, every thing has to be arranged in 

 sequence, and therefore in the linear mode, 

 now so generally followed by naturalists. In 

 this way, however, it is impossible to express 

 the enchainment, the inosculation, so to speak, 

 of the different groups. To counterbalance this 

 inconvenience, we are of opinion that the clas- 

 sification ought to be made with lateral offsets, 

 now terminating abruptly, now divided once or 

 twice, sometimes inosculating variously, ani 

 again, departing from a common trunk, dispo- 

 sed in one case in a right line, in another in a 

 curved line, and in a third in a circle. We 

 conceive that it is according to these new views 

 only that the acephalous mollusks can be pro- 

 perly arranged ; it is accordingly upon the 

 principles just announced that the following 

 table is constructed. 



Although in the present stale of our know- 

 ledge of these animals many important parti- 

 culars are still unquestionably wanting, this 

 division of the molluscous tribes nevertheless 

 presents fewer gaps than any of the others, in- 

 asmuch as opportunities have occurred of ex- 

 amining some one or other of the animals be- 

 longing to the whole of the genera. 



