CHAP. II. ANALOGIES OF THE TESTACEA. 49 



Analogies of the Testacea to the Vertebrata. 



.. Orders of the Testacea. Circle of the rcrfebrata. 



Gasteropoda.. Quadrupeds. 



UiTiiYRA. Birds. 



NuniBRANCHIA. ' ReFIILES. 



ParExNchymata. Amphibians. 



Cephalopoda. Fishes. 



(40.) The reader will bear in mind that, hitherto, we 

 have spoken of relations of affinity ; hut our present 

 business is with analogies, or mutual representations. 

 Setting aside, therefore, all we have already advanced, 

 we accordingly find that the best modern zoologists place 

 the Dithyra, or bivalve shell-fish, close to the Gaste- 

 ropoda, or univalves, upon the same principle as the 

 birds, in the other column, follow the quadrupeds. 

 This arrangement, at the very first, seems to carry with 

 it an appearance of being natural, because we thus find 

 that both the Dithyra and the Aves are the sub-typical 

 divisions of their own circles. But this parallelism, 

 although highly satisfactory, is not of itself sufficient ; 

 we must look further, and inquire whether these groups 

 actually agree in any particular circumstance of struc- 

 ture, common to both. Now, every one knows that, 

 among the many things in wdiich quadrupeds differ 

 from birds, their mode of feeding is altogether dissi- 

 milar. Quadrupeds are provided with jaws bearing 

 teeth, with which the food is masticated before it enters 

 the stomach : birds, on the contrary, have the jaws 

 dilated into a snout-shaped bill, and teeth are alto- 

 gether wanting. The rapacious birds (Raptores), in- 

 deed, which typify the quadrupeds, may be said, in some 

 degree, to masticate their food ; but by all others it is 

 sucked into the mouth, the effects of mastication being 

 supplied by the stomach. If we inquire into the modes 

 in which the typical Gasteropoda and the Dithyra take 

 their food, we find that it is precisely conformable to the 

 above variation. The most typical of the spiral shell- 

 fish (^Gasteropoda^ like quadrupeds, are provided with 

 fleshy lips and corneous jaws, varied, as M. Cuvier truly 

 observes, into numerous modifications ; v^hile in many 

 genera (on the same- authority), the inside of the cheeks 



