282 MR. BELL, HORÆ CARCINOLOGICÆ ; 
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 Persephona, 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 unidentata, 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 “ incisio 
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 vith 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 
Crustacés,’ two species only of this genus were distinguished, L, Urania and craniolaris. 
