26 SHELLS AND SHELL-FISH. PARTI. 



(22.) In both these latter methods of classifying the 

 Testacea, there is much to approve and much to dissent 

 from. It is clear that neither of these naturalists., how- 

 ever great their merits undoubtedly are in other respects, 

 have paid any attention to the difference between analogy 

 and affinity : nor have they aimed at anything beyond 

 producing a simple scale or line of connection from one 

 group to another. Now, as no such simple series exists 

 in nature, whose relations of affinity are always 

 double, and generally treble, and whose analogies are 

 interminable, it follows, as a matter of course, that 

 both have completely failed in laying down a simple 

 graduated scale of the objects before them. Hence, as 

 the groundwork of both systems is founded in error, 

 no apology is necessary for rejecting them, particularly 

 when the object sought for is the discovery of the na- 

 tural arrangement. It is time, however, to leave the 

 systems of others, and to lay before the naturalist some 

 details of that here proposed. 



CHAP. II. 



ON THE TESTACEOUS MOLLUSCA IN GENERAL, THE PRIMARY 

 DIVISIONS, AND THEIR ANALOGIES TO THE VERTEBRATA AND 

 ANNULOSA. 



(23.) THE testaceous mollusks, or shell-fish, with few 

 exceptions *, are all marine, or, at least, aquatic animals, 

 soft and slimy in their nature, and without articulated 

 limbs, so that such as can move about, crawl upon 

 their belly, or swim in the water by means of the fin- 

 shaped lobes of their mantle. Independent of their 

 peculiar anatomical structure, and merely looking to 

 external characters, the most perfect of the testaceous 



* Some of the parasitic groups, and the land shells, or slugs. 



