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, aril 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 state 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.
This tabular view of the classification exhibits certain particulars, upon which we deem it necessary to offer a few explanatory remarks. As we said before, the series as a whole may be regarded as a common trunk, from which various branches spring, sometimes anastomo sing, sometimes ending abruptly. It is thus that from the Clavigella vve observe a lateral line departing, formed of the genera Fistulana, Galeomma, and those of Lamarck's family of Petricola. These genera descend parallel to the common trunk of the classification, so as to approximate in as great a degree as possible the genus Venerupis to tbe genus Venus. The genus Pandora has numerous analogies on one side with the Corbula, but it has also many with the members of the genus Osteodesma, on which account it is made to depart laterally from the Corbula, and to ascend towards the Osteodesniata. The Lutraria are also variously related to several genera of the Osteodesmata, and this genus is joined to that of the Thracia by means of the genus Anatinella, which we place crosswise to connect the genera just men tioned. In the Mactracea, we pass without
any very great stride from the Lutraria to the IVIactra, from the Mactra to the Erycina and to the Amphidesma. Farther, in order not to interrupt this series of relation ships, we place upon a lateral line departing from the 'Mara the two genera Mesodesma and Crassatella. Every naturalist knows how great the resemblance is between the flat and broad Solens ( Soletellina, De Blainv.) and the Psammobice ; but we also know that the genus Psammobia has so many analogies with the family of the Tellinida, that it is impossible to detach it from this family in order to include it within the family of the Solenaceaa. To avoid interrupting tbe relations of this genus to those of the Solen family, we have recourse to an ascending line composed of the genera Solen ertus, Panopea, Solen, Solemya, and Glyci merts, by which means we approximate, as much as possible, these last genera to the fa milies Pholadia and Osteodesmata, with which they have in fact unequivocal relationships in point of organization. We consider the family of the luciniche as a lateral and truncated branch of the Conchidee, divaricating from the genus .Astarte. With regard to the Cyclada, we place the genus Glaucoma of Mr. Gray laterally, between the Cyrenas and the Venuses, so as to establish the connexion between the two ge nera; whilst departing also from the genus Cyrena we place our genus Cyrenella obliquely in order to make it join that of Lucina, this genus of Cyrenella being to Cyrena and Lucina that precisely which the genus Glaucoma is to Cyrena and Venus. To us the family of the Chamacea is a lateral offset from that of the Cardiacea, and although the Etheria and the family of the Rudistes are in reality among the number of those Conchife rous mollusca which have the lobes of the mantle disjoined, still as they do not imme diately arrange themselves in any particular part of this section, we have placed them to the side in continuation of the family of the Cha macea, but underneath them. The family of the Ostracea we now believe to consist of the single genus Ostrea, and we propose under the name of Placunidee a family containing the three genera P lacuna, Placunonomia, and Ano mia, which according to our views constitute a descending and lateral line really intermediate to the Conchifera and the Brachiopoda.