PORCELLANOPAGURUS— BORR AD AILE. 123 



the eggs, in Ostraconotus ; simple,* slender, and unusually small in Ti/laspis. The 

 walking legs (pairs 3 and 2) in Ostraconotus have very remarkable flattened dactylo- 

 podites that almost suggest a swimming function ; in Tylasjyis 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 difl^erent 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 Lithodiuae from Eupagurinae, 

 which differed from Eupaguras 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, l^ut 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 be 

 traced from a decapod which, though its structm-al 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. 



t The evolution of this group is discussed by Bouvier, Ann. Sci. Nat. (7), XVIII, p. 1.57 (189.5). 

 X See Ann. Mag. Nat. Hist. (7), XIX, p. 473 (1907). 



