Mr. C. Spence Bate on the British Diastylidse. 451 



means we shall be enabled homologically to consider the relation 

 which their several parts hold to the same respectively in the 

 larvse of the Decapoda, and demonstrate not only that the Cuttkb 

 are not the young of certain Macroura, but that they are animals 

 complete in their development and capable of the production 

 of others of their own form. 



That the Diastylidce are a depauperized family, there can, I 

 think, be no doubt; yet it is one of those forms in creation 

 which assist to destroy the popular theory of authors, of a gradual 

 rise in the gradation of animal existence ; for though in classifi- 

 cation we place them among the higher types, yet there can be 

 little doubt that in organized perfection they are less complete 

 than those of animals below them in the natural scale of ar- 

 rangement. 



Genus Diastylis. 



Diastylisy Say, Trans. Phil. See. Philad. vol. i. 

 Alauna, Goodsir, Edin. New Phil. Journ. (1843). 

 Cumay Kroyer (Voyages en Scand. &c.). 



Carapace with the lateral angles developed anteriorly, and 

 meeting without uniting in front of the eye and antennal seg- 

 ments, and produced anteriorly in the form of a rostrum. Eyes 

 confluent, and situated as a single organ on the top. Five seg- 

 ments of the thorax exposed behind the carapace. Upper an- 

 tenna short, scarcely reaching to the anterior margin of the 

 carapace. Lower antenna longer than the upper. First five 

 abdominal segments without appendages, except the two anterior 

 in the male only. The sixth furnished with a pair of members 

 terminating with double stylets. The telson^ produced into a 

 long styliform process. 



Diastylis Rathkii. PI. XIII. 



Cuma Rathkiiy Kroyer. 

 Alauna rostrataj Goodsir. 



The genus Bodotria is perhaps the highest form in the family ; 

 but since my opportunity of dissection has been more complete 

 on the Alauna of Goodsir, which I believe to be of the same 

 genus as Diastylis of Say, I shall take this latter as the type 

 of the whole family, and under their respective heads trace the 

 generic or specific diff'erences in the group. 



The first character in the general appearance of one of these 

 animals that strikes the observer is that of its being a muti- 

 lated creature, — an idea present to the mind of Montagu when 



* From reXa-ov, extremity. The centre tail -piece in Crustacea generally ; 

 the twentv-first segment in the homologies. 



29* 



