38 DR. G. C. BOUUNE ON THE EANINIDjG : 
epimeral walls iu this area, being exposed, are strongly calcified. This 
exposure of a large portion of the epimeral surface is a remarkable and 
unique feature in the Raninidse. Even in the Astacura, where the lower 
edge of the branchiostegite is free, it reaches down to the bases of the limbs, 
and in all crabs the lower edge of the branchiostegite is so closely adapted 
fo the coxa} of the thoracic limbs as to leave no passage for water into the 
branchial chamber behind the chela3. This peculiarity did not escape the 
attention of Milne Edwards, but subsequent authors seem to have paid very 
little attention to it. If we now examine the interior of the thorax, we find 
that the apodemes between the ventral and exposed parts of the three 
thoracic epimera in question are shallow and form nothing more than two 
inconspicuous thickenings internally, strengthening the framework of this 
part of the flanks but not projecting into the cavity of the body. But the 
npodemes above the line of the adherence of the branchiostegites are very 
deep and form well-marked endopleurites which, as usual, bifurcate and their 
branches unite with the expanded summits of the endosternites next in front 
and next behind them. It appears that the excessive depth of the sternal 
canal is correlated with the restriction of the endopleurites to the upper 
moieties of the epimera or, to put it in another way, with the downgrowth of 
the ventral edges of the epimera beyond the line of the attachment of the 
branchiostegite. The ventral extension of the epimera is an adaptive 
feature, and receives its explanation when the habits of the animals are taken 
into account. The Raninidse are digging crabs, with large and powerful 
chelipeds and first, second, and third pereiopods. In many the fourth pair 
of pereiopods is weak and slender, though in Ranina this last pair is not 
much weaker than the others. The powerful digging limbs imply powerful 
abductor muscles and large adductor muscle-cavities. Reference to fig. 9 
shows that these cavities are in fact unusually deep, and their extent is due 
to the great length of the endosternites within the body and to the large 
ventral extensions of the epimera forming their outer walls. The same 
figure shows that, as in crabs, the abductor muscle-cavities lie above and not 
alongside of the adductor muscle-cavities. But cancroid as this character 
is, it has clearly a verj' different origin from the similar arrangement 
in crabs. 
To turn to another feature noted bj' de Haan, " thoracis interior offert 
sellam turcicam uti tantum in Brachyuris." What is this sella turcica ? 
The name is due to Audouiri and Edwards, and the latter authoi- has 
described tlie elements entering into its composition with great minuteness 
on p. 34 of the first volume of the ' Histoire Naturelle des Orustaces.' 
Paraphrasing his description, the sella turcica posterior is a little arch or 
vault, concave from side to side and convex from front to back, formed by 
the forward extension and fusion together in the middle line of the nearly 
