PORCELLANOPAGURUS—BORRADAILE. 
P23 
the eggs, in Ostraconotus ; simple,*' slender, and unusually small in Tylaspis. r Flie 
walking legs (pairs 3 and 2) in Ostraconotus have very remarkable flattened dactylo- 
podites that almost suggest a swimming function ; in Tylaspi s they are very long and 
slender ; in Porcellanopagurus little modified from those of an ordinary hermit-crab. 
The chelipeds are of quite different types in all three, as inspection of the figures 
will show. In short, there is not the least resemblance between the three cases, and 
when all the facts are known, there is little doubt that it will appear that the crab¬ 
like habit of body has arisen in different circumstances, and is made viable by different 
modes of life, in all of them. I have indicated the explanation of the case of 
Porcellanopagurus. In the other two genera there is great likelihood that the soft 
abdomen is somehow protected in life. Perhaps, as they are both deep-water animals, 
it is merely buried in the ooze of the sea floor. Certainly in Ostraconotus it is not 
carried under the cephalothorax, and its unarmoured dorsal side makes it unlikely that 
this is the case in Tylaspis. 
Superficially, the abdomen of Porcellanopagurus resembles that of Birgus more 
than that of any other pagurid, but the position of its egg-bearing limbs is different, 
and in any case Birgus belongs undoubtedly to the Pagurine stock, while Porcellano¬ 
pagurus and the other genera we have been discussing are as certainly Eupagurine, 
so that there can be no question of relationship in this case. 
The Lithodidae.f with their flat, hard-backed abdomen, deprived of uropods and 
pressed against the sterna of a very crab-like cephalothorax, present a more advanced 
case of the carcinization of Paguridea than those we have hitherto mentioned, but 
there appears no likelihood that any of them are connected with those less highly 
modified forms. They are, in truth, probably diphyletic, the Lomisinae being derived 
from primitive, trichobranchiate Pagurinae, and the Lithodinae from Eupagurinae, 
which differed from Eupagurus in keeping a pair of limbs on the first abdominal 
segment of the female, although they had lost that feature in the male. They must 
therefore have left the Pagurid stock at a point not very far removed from that at 
which Porcellanopagurus took origin, but there is no possibility of reconciling the two 
cases in the crucial matter of the course of evolution of the abdomen. 
Still less, of course, can the Hippidea, the Porcellanidae, or the true crabs, all 
primarily symmetrical groups, be supposed to have arisen either from a hermit-crab— 
or, for that matter, from one another. The descent of the true crabs, indeed, must lie 
traced from a decapod which, though its structural features would bring it under the 
Anomura, as that group must be defined,J was more primitive than any existing 
member of the tribe. 
* At the end of the propodite of the fourth leg of Tylaspis (Fig. 12) there is a slender process, but 
this is not in the plane in which the dactylopodite works, so that there is no chela. 
f The evolution of this group is discussed by Bouvier, Ann. Sci. Nat. (7), XVIII, £>. 157 (1895). 
1 See Ann. Mag. Nat. Hist. (7), XIX, p. 473 (1907). 
