714 CONCHIFERA. 



produced, as we have already said, by the pro- contrast. Better data might perhaps be ob- 



per retractor muscle of the siphons. tained from the conformation of the organs of 



Besides the muscular impressions of which digestion ; but these organs have hitherto been 



we have now spoken, several others of much examined in comparatively so small a number 



less importance have been particularized in the of genera and species that they cannot be 



greater number of the conchiferous mollusks. brought forward usefully in supplying cha- 



All the species that have a foot have peculiar racters for a general classification. If, as we 



muscles to move this organ, and these have ourselves feel inclined to do, the hinge be 



their fixed point of action on some point of taken as the point of starting in the Pholades, 



the interior of the shell. They are generally this part may be made the means of giving 



divided into two principal fasciculi ; the one excellent characters in its principal modifica- 



runs to be inserted within the hooks, the other tions for the establishment of genera. It is, 



in the Dimyaria proceeds to be attached before indeed, very remarkable that we should find 



and above the posterior adductor muscular the characters as indicated by the hinge 



impression. In the Moriomyaria, the foot of almost constantly in harmony with those af- 



which is generally rudimentary and without forded by the rest of the organization ; and 



use, we observe nothing more on each side with a few exceptions, relative to several ex- 



of the body than a single small fibrous fas- tremely natural families, that of the Unios for 



ciculus, the impression of which is found on example, all that is valuable in the generic cha- 



the inside of the hooks. In some genera of racters generally may be preserved along with 



Dimyaria, and particularly in the Unio, we the characters supplied by the hinge. Ano- 



observe three and sometimes four muscular im- ther character which may be usefully employed 



pressions belonging obviously to the adductor in classification is assumed from the regularity 



muscles of the valves, which are occasioned or irregularity of the shell of the animal ; in 



by the anterior adductor muscle in particular generalizing upon this, like groups are obtained 



being divided into two fasciculi, often of un- in the two principal divisions of the Conch ifera, 



equal size, as in certain Uniones, and some- and the two principal divisions of the classi- 



times equal and of considerable magnitude, as fication are referred to the simplicity or exact- 



among the Iridines. ness of the dichotomy, whilst natural groups 



From the summary and concise view we are preserved as much as may be in the linear 



have taken of the principal facts in the organi- arrangement. 



zation of the Conchifera, very important con- Method, it must ever be remembered, is an 



elusions may be drawn with reference to the artificial means of introducing order among a 



classification of these animals. series of observed facts, arid of approximating, 



Taking the Conchifera, properly so called, according to the analogy of their organization, 



and looking narrowly into that which is of the beings which nature has scattered over the 



most importance in their organization the ner- face of the earth ; method is a human creation 



vous system, we find two principal modifica- altogether, and in this light must it be viewed, 

 tions, coinciding in a very remarkable manner To be all it ought, every known fact must be 

 with the number of the muscles. This num- included, and the greatest possible amount of 

 ber of the muscles, permanently proclaimed organic relationships between the individuals 

 by the impressions they leave on the shell, of each great class must be indicated. In an 

 presents an important character by means of exposition of facts seriatim, and as they occur 

 which, while we define their limits somewhat in a book, every thing has to be arranged in 

 more strictly, we feel authorized in retaining the sequence, and therefore in the linear mode, 

 two grand orders of Lamarck, the Conchi- now so generally followed by naturalists. In 

 fera Dimyaria, and' the Conchifera Mono- this way, however, it is impossible to express 

 myaria. A fact of some importance, and the enchainment, the inosculation, so to speak, 

 brought to light by the observations of Poli, of the different groups. To counterbalance this 

 is that a small nervous ganglion exists at the inconvenience, we are of opinion that the clas- 

 point of commissure in those acephalous mol- sification ought to be made with lateral offsets, 

 lusks which have the lobes of the mantle con- now terminating abruptly, now divided once or 

 joined. This peculiarity gives new conse- twice, sometimes inosculating variously, and 

 quence to the characters drawn from the con- again, departing from a common trunk, dispo- 

 joined or disunited state of the lobes of the sed in one case in a right line, in another in a 

 mantle. Unfortunately the circumstance is curved line, and in a third in a circle. We 

 not always indicated upon the shell ; it is, in conceive that it is according to these new views 

 fact, only obvious upon those inhabited by only that the acephalous mollusks can be pro- 

 siphoniferous animals; it is quite inapprecia- perly arranged; it is accordingly upon the 

 ble upon those the inhabitants of which have principles just announced that the following 

 siphons so short as not to require a particular table is constructed. 



retractor muscle to draw them within cover of Although in the present state of our know- 

 tlie shell. With regard to the other organic ledge of these animals many important parti- 

 characters which furnish data available in clas- culars are still unquestionably wanting, this 

 sifying the Conchiferous mollusks, these are all division of the molluscous tribes nevertheless 

 of so little permanency that they are only presents fewer gaps than any of the others, in- 

 useful in supplying secondary hints for the asmuch as opportunities have occurred of ex- 

 arrangement of families and genera. Thus amining some one or other of the animals be- 

 neither the branchiae nor the heart present any longing to the whole of the genera, 

 character susceptible of generalization or of 



