found on the South Coast of Devonshire. 105 



The body is inequilaterally ovate*, composed of thirteen flat 

 joints, the articulations forming as many scallops on the sides: 

 antennae and eyes obscure : legs fourteen, very short, crook- 

 ed, and usually folded up and concealed under the seven 

 first, or anterior scallops on each side: the under part of the 

 body between the legs is covered with broad membranes that 

 collapse and form a receptacle for the eggs, which are extremely 

 small and numerous. 



Length rarely half an inch. 



Colour pale greenish, and glossy above ; the abdominal mem- 

 branes dark at their edges. 



The most incurious cannot but have noticed the tumour so 

 common on the thorax of the prawn or shrimp during the sum- 

 mer months, that is occasioned by the lodgement of this animal, 

 whose growth occasions the distortion of the shell. This tumour 

 forms a secure asylum for the protection of the more than usu- 

 ally soft and membranaceous bodies of these parasitical Onisci. 



That an insect so extremely common, and obvious to the most 

 cursory observer, should not have found a place in the Sifatema 

 Natura appears very extraordinary ; and I have been induced 

 to describe it, because it seems to have been omitted by even 

 the more modern systematists ; or is certainly misplaced, and 

 not where it ougM to be. 



The male, which has hitherto escaped observation, is probably 

 very minute, as in the preceding species. 



« 



* The dissimilarity in the sides of these insects is occasioned by the unequal pressure 

 they receive from the thoracic shell of the crab j this inequilateral growth is therefore 

 not constantly alike, but depends on the side of the thorax each individual inhabits. 



VOL. IX. 



MOLLUSCA. 



