18 TESTACEA. 



Amphitrite or Terebella : 2. Those analogous to the Limax or the Doris : 

 3. Those to be compared to the Ascidia : 4. Those corresponding with the 

 Tethvs, a creature said to live free in the warmer climates, of which I can 



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speak only on the authority of others, none without the shell being 

 recognised as inhabitants of Scotland. 



These are the principal animals of the Scotish Testacea. But there 

 is very little uniformity among them ; and although by generalization 

 we combine the Ascidia with extensive genera, we shall find it vain to 

 attempt identifying the species, from the great modifications of organiza- 

 tion and aspect individually presented by them. 



Neither do many strictly correspond with the animals taken as a 

 type of the genus. Naturalists are perplexed in determining the pre- 

 cise distinctions of several, such as the tenant of the Dentalium, and of 

 the Chiton. 



The univalve Testacea, dwelling in the Scotish waters, are not un- 

 common. In some lakes and ponds particularly, they are extremely 

 numerous. On the contrary, the bivalves are in no variety, and they 

 are extremely rare. 



Very few bivalves comparatively belong to the fresh- waters of any 

 country in Europe, but their number seems reduced very low in Scotland, 

 insomuch that it is with some reluctance that any are presented before 

 the reader. 



The relation of the animal to the shell, mast regulate those which 

 shall be included with the Testacea. On this score, perhaps the Serptila 

 and Dentalium are inhabited by animals which should be excluded. 



Naturalists do not consider the shell as constituting distinctions 

 among the genera of their inhabitants. Hence, perhaps, may appear the 

 futility of the systems of Conchology, derived from the form of the shell 

 alone. 



But, however interesting many shells are, undoubtedly to the spec- 

 tator, merely from form and beauty, such a system of Conchology cannot 

 be admitted as a portion of the Systema Natura, for it would lead to very 

 incongruous associations. 



These facts, indeed, are of less consequence here. The Testacea did 



