Classification of the Thalassinidea. 535 
cases the information available has not been enough to enable 
me to place them satisfactorily. It follows, of course, that 
the diagnoses of the genera can only be approximately correct, 
and are liable to be altered in detail by the inclusion of 
other species. No doubt the discovery of new species will 
have the same result. The chief character on which our 
knowledge of the recorded forms is defective is the gill- 
formule, and, while the systematic value of these has been 
somewhat overrated, there can be no doubt that it is con- 
siderable and that they afford useful guidance in the more 
primitive family Axiide. In the Callianasside, on the other 
hand, and particularly in the Upogebiine, they will, I think, 
be found to give less help, since they are here more constant. 
The Thalassinidea are a group of tailed Decapods which 
recall the hermit-crabs in some respects and the lobsters and 
crawfish in others. They are like the Nephropsidea in the 
shape of the tail-fin and of the first and often also the second 
leg. They differ from them in never having the third leg 
chelate, often in a reduction of the number of their gills, and 
in a tendency of their abdomen to become soft and lose its 
pleura. This is to be connected with their mode of life, 
which is in most cases a burrowing one. Herein they show 
the same habit of concealment as the Paguridea, to which 
they are also akin in the other points of difference from the 
Nephropsidea already mentioned, in the thorn-like shape of the 
antennal scale in such of them as have it well developed, in 
the freedom of the last thoracic sternite, and in their peculiar 
way of carrying the last pair of legs rather apart from and 
above the rest. They differ from the hermit-crabs in the 
fact that these legs are nevertheless shaped much like the 
rest, that their second pair of legs are usually chelate, and 
that their abdomen is symmetrical with a broad tail-fin. A 
remarkable feature, which recalls the prawns, is the presence 
in most of them of an appendix interna on the abdominal limbs. 
They may be divided into four families—Axiide, Laomediide, 
Thalassinide, and Callianasside. The characters and sub- 
divisions of these are set forth below, and will be found 
summed up in key form on p. 549. 
G. West America, below California, 
H. North Pacific to California and Vladivostock. 
I, Indo-Pacific. 
Of these divisions, A, B, and H together make up Ortmann’s Arctic 
region, while D and F are his Antarctic region. ‘The deep- and shallow- 
water forms are arranged by the same set of regions. 
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