Chilton. — On some Subterranean Crustacea. 175 



The Isopod that I have to describe is remarkable from the fact that it 

 has only six pairs of legs, whilst the normal number is seven pairs. In 

 many Isopoda the young have at first only six pairs of legs, the last thoracic 

 segment being but slightly developed and destitute of appendages,* and 

 hence it might, at first sight, be thought that the animal I have is only an 

 immature form. This, however, I think can hardly be the case, for there is 

 nothing embryonic about the appearance of the animal, and moreover, I 

 have examined altogether twenty-two specimens, varying in length from -16 

 of an inch to -46 of an inch, and these all agree in wanting the last pair of 

 thoracic legs. These specimens were obtained at various times from 

 January up to October, 1881, and I think it is hardly possible that these 

 can all be immature forms, and that dm-ing the whole time not one mature 

 form should have been obtained. If it is, therefore, a mature form, the 

 absence of the last pair of thoracic legs must I suppose be due to arrested 

 development. 



ISOPODA. 

 Cruregens, (nov. gen.) 



Generic characters : — Body sub-cyUndrical. Head small. First six 

 thoracic segments sub-equal, the seventh small and without appendages. 

 Antennae sub-equal, neither having a flagellum. First pair of thoracic legs 

 large and sub-chelate, the second and third sub-chelate but smaller, the 

 three posterior pairs simple. First pair of abdominal appendages forming 

 an operculum enclosing the branchial plates ; last pair biramous. Telson 

 squamiform. 



In the antennae, the shape of the body and of the thoracic legs, this 

 genus resembles Paranthura, Spence Bate, and in the shape of the telson 

 and the last pair of abdominal appendages, it is like the closely alhed genus 

 Haliophasma, Haswell, but it differs from both in the absence of the last 

 pair of thoracic legs. 

 Cruregens fontanus, sp. nov. PL X. figs. 1 to 12. 



Eyes none. Short blunt rostrum between the bases of the upper 

 antenna. Upper antennae slightly shorter than the lower, formed of four 

 joints ; first joint of lower antennae long, second short, third and fourth 

 about as long as the first, the fourth being followed by a short terminal 

 joint. First pair of thoracic limbs strong and sub-chelate ; hand large, 

 broadest at the proximal end, narrowing distally, che palm armed with 

 stout spines, the finger strong and shghtly curved ; the wrist about twice 

 as long as broad. Last pair of abdominal appendages two-branched, first 

 branch consisting of a single long narrow joint, the other of two joints, the 



• " Facts and Arguments for Darwin," Fritz Miiller, pp. 70-72. 



