A STUDY IN CARCINOLOGY. 53 
to enumerate tliem, but the labour may be spared because the majority o£ 
them could plausibly be explained on a tiieory of descent with modification 
from a Raninid ancestor. This explanation could be given satisfactorily in 
such matters as the concentration of the nervous system, the disappearance 
of the sternal canal, the broadening of the posterior thoracic sterna, and a 
large number of other characters. But it cannot apply in the case of 
structural features which have been profoundly altered or have disappeared 
altogether in the Raninidse but are present and exhibit normal relations in 
Oalappidse and Leucosiidse. 
To take first a feature peculiar to the Raninida?, the marked reduction in 
vertical depth of the posterior part of the branchiostegife whereby a con- 
siderable area of the epimera of the eleventh, twelfth, and thirteenth 
segments is left uncovered. This is by uo means primitive but a definite 
specialisation, and I have attributed it to the burrowing habits of the f;imily. 
The Calappina3 are certainly and tlie Matutinte largely sand-burrowing 
criibs, butin l)oth the lower edges of the branchiostegite fit very closely to the 
coxa3 of the pereiopods. In the Leucosiida3, which are supposed to resemble 
the E.aniaida3 more closely than other Oxystomes, the adaptation of the lower 
edges of the branchiostegite to the goxeb of the pereiopods is particularly' 
close and elaborate. It cannot be argued that the original and more 
primitive relations of the branchiostegite to the epimera were re-established 
when the necessity for enlarged muscle-cavities for the pereiopods disap- 
peared with the assumption of new habits by the Calappidse ;ind Leucosiidge, 
for the muscle-cavities are very large in these families but their enlargement 
is provided for in a very different manner. In the Leucosiidse the arrange- 
ment of the elongated abductor muscle-cavities is peculiai- and interesting, 
but there is no room to describe it in this place. 
lu all the Raninidaj the posterior margin of the pterygostomial region of 
the carapace is closely united to a broad offset of the tenth sternum in front 
of the cheliped. Consequently there is no inhalant branchinl orifice in 
front of the cheliped, and the epipodite of the third maxilliped is aborted, 
though a trace of it remains in the form of a vestigial setobranch. In the 
Calapi)in8e and Matutinse the pre-chelipedal inhalant aperture is conspicuous 
and the epipodite of the third maxilliped well developed. It cannot seriously 
be maintained that these structures have been re-acquired in these two sand- 
burrowing sub-families. In the Leucosiidse, however, the posterior margin 
of the pterygostome is as intimately fused with the plastron as in the 
Raninidse, and with the disappearance of the pre-cheliped;d branchial orifice 
the epipodite of the third maxilliped has disappeared even more completely 
than in the Raninidse, for there is not even a vestigial se(obranch. A similar 
state of things occurs in some of the Cyclodorippidfe, but need not be 
discussed here because the Dorippidas have already been excluded from 
