282 MR. BELL, HORJE CARCINOLOGIC^ ; 



species than any other. Its characters are so defined, that none of the species exhibit any 

 approximation to another generic form. The genera Myra and JPersephona, and Ilia and 

 Leucosilia, have so much that is common to them all in their structure and general aspect, 

 that it requires close examination to determine their limits ; but in Leucosia, numerous 

 as the species are, not only the essential characters of the parts on which the generic 

 distinction usually depends, but also the general features of the whole form, are remarkably 

 similar in all the species, and obviously distinct from all others. 



There are indeed in this genus many interesting points in the general form and colour 

 and other less essential characters, which although not of much physiological or functional 

 import, are curious as exhibiting coincidences which in many cases serve to indicate a 

 close relationship as strongly as those modifications of organs and functions which have 

 a more important bearing upon the habits of the species, and are commonly considered as 

 essential. The polished carapace ; — the absence of all hairiness or other clothing, which 

 is universal, with the exception of two species, L. Whitei and umdentata, in which a small 

 quantity of close hair exists amongst the tubercles of the arms ; — the existence of large and 

 distinct granulations, or rather tubercles, generally distinctly coloured, upon at least the 

 proximal part of the arm, extending forward in lines more or less numerous ; — the tendency 

 to a brown or purplish-brown colour in most of the species ; — and the occurrence, in very 

 many, of spots occupying the same situation on the carapace, four of which are paler 

 than the ground colour, and placed at the anterior part on the gastric region, and two 

 dark ones upon the posterior portion of the branchial ; — these are amongst the characters 

 of minor consequence to which I have alluded, and which, without having any bearing 

 upon function or habit, are interesting from the constancy or frequency of their occurrence, 

 and as indicative of certain structural or formal tendencies throughout the whole genus. 



There is also one remarkable peculiarity which exists in almost all the species of Leu- 

 cosia, but I believe in no other genus, the variations of which constitute good specific 

 characters. This is a sinuous groove running along the side of the thorax, bounded above 

 by the lateral margin of the carapace, and beneath by the upper edge of the epimeral 

 plate ; the former loses itself in the posterior part of the branchial region, the latter is 

 continued into the posterior margin of the carapace. It commences in front of the first 

 pair of legs, where it forms a single or double notch, or a deep, almost circular cavity. 

 Its margins are usually tuberculated, and the tubercles are ordinarily largest at the ante- 

 rior part. A very few species, as for instance L. orbicularis, have no such groove, the epi- 

 meral piece or space between the lateral margin and the junction of the carapace with the 

 sternum being plain and smooth. De Haan is the only author who has noticed this 

 curious structure, and he has only mentioned its anterior limit, which he terms " memo 

 ante chelarum insertionem." This expression, however, gives no idea of its true extent or 

 course ; and I have called it " sinus thoracicus," as more expressive of its character, and 

 more easily available in specific description. Of its use I can offer no suggestion. It does 

 not appear to have any connexion with the interior of the body, nor can it be supposed to 

 have any important office, as in some species it does not exist. 



In the year 1837, when Professor Milne-Edwards published his ' Histoire Naturelle des 

 Crustaces,' two species only of this genus were distinguished, L. Urania and craniolaris. 



