48 SHELLS AND SHELL-FISH. PART I. 



the VolutidcB. The transition from these to the CyprmdcB 

 is no less clear than that from the Cyprceidce to the Bullce, 

 — the latter being the most perfect of the Tectibranchia. 

 In some of these half naked gastropods^ the foot-like 

 disk of the belly is reduced to very small dimensions, 

 while the two lobes of the mantle become so dilated 

 that they are used as fins. In this manner does nature 

 leave the Gasteropoda^ and unites them to the swim- 

 ming order of Cephalopoda_, — the first tribe of which is 

 the Pteropoda. The straight cylindrical case of some of 

 these singular animals, few in number, but highly in- 

 teresting, prepares us for that extinct tribe of cuttle- 

 filsh of which the fossil Belemnites, in all probability, 

 formed the type : from these to the spiral Cephalopoda, 

 the passage is so easy, that it can be traced by the 

 shells alone; and then, with the assistance of Argonauta 

 and Carinaria, we at once reach the Heteropoda. The 

 affinity between some of these simply constructed 

 animals and the aberrant Parenchymata has already 

 been intimated. Thus we find that the two lateral 

 series which branch off from the Planaridcs meet toge- 

 ther in the Turhidce, and complete the circle of the 

 whole class. 



(39.) In thus tracing the course of the circle of the 

 Test ACE A, the zoologist, versed in the circular theory, 

 will perceive we have taken no notice of the most aber- 

 rant groups in each order, — and this, because it would 

 have incumbered, without necessity, the simple ex- 

 position of the union of the orders themselves. He will 

 also be aware that it is by these particular groups that 

 each order forms a circle of itself. This important pro- 

 perty will, of course, be adverted to when we come to 

 treat of each order separately. In the mean time, we 

 shall now compare the orders of the Testacea with such 

 other groups of the animal kingdom as appear calculated 

 to render their analogies more comprehensible to the 

 ordinary reader. Our first comparison wiU consequently 

 be with the vertebrated animals. 



