8 THE VOYAGE OF H.M.S. CHALLENGER. 



This group may be separated into two divisions, — first, in which the animal is dorso- 

 ventrally compressed ; second, in which the animal is laterally compressed. 



The first division corresponds only with the tribe Galatheides of Milne-Edwards's 

 Division, Macrura Cuirasse's, and part of the Loricata of Heller. 



The second division corresponds with the tribe Macrura Fouissures or Thalassiens of 

 Milne-Edwards, Thalassinidea of Dana, and Thalassinidae of Heller, and contains 

 several families, which, while they have a character that is common to all, yet possess 

 features that are extremely at variance with one another in very closely affiliated forms. 



Their structural relations assimilate them to the Anomura, and where they 

 depart from that resemblance, they do so by approaching the condition of immature 

 forms. The genus Pomatocheles, like the Paguridaj, inhabits molluscous shells, and 

 possesses all the external characters of an Anomurous Crustacean, and Pylocheles was 

 taken dwelling in the hollow of a mass of indurated sand. These facts induce the belief 

 that Clieiroplatea may also reside in some dwelling-place of its own selection. In this 

 latter genus we see a, close resemblance in the cephalic appendages to those of the 

 Anomurous form in the genus Cenobita, whereas the rest of the animal approximates to 

 the character of the immature stage of Pagurus described by Milne-Edwards under the 

 name of Glaucotlwc, with the exception that, while Glaucothoe exhibits evidence of a 

 tendency to bilateral variation, Clieiroplatea, Pylocheles, and Pomatocheles are perfectly 

 symmetrical. The same remarks may also be applied with perhaps less force to the genus 

 Thalassina, which approximates to Pagurus, as the previous genera resemble Cenobita. 

 The branchiae are variable in this group, but with a tendency, more or less complete, 

 to the trichobranchiate condition ; in some genera, as in Thalassina, they are both 

 foliaceous and filamentous ; in some filamentous and cylindrical, as in Clieiroplatea ; in 

 others filamentous and compressed, or flattened, as in Eiconaxius, with a tendency, 

 where the pressure is less complete, to return to the cylindrical condition. 



Callianassa retains all the external features of an Anomurous Crustacean, but is 

 modified from the younger form which approaches the Macrurous type ; this is most 

 constantly exhibited in the tendency of the posterior two pairs of periopoda to undergo a 

 variation from the original simplicity and normal use. 



All carcinologists following Milne-Edwards classify the genus Callianidea not only 

 in a separate family but also in a distinct group, forming the tribe of the Gastrio- 

 branchides of Milne-Edwards, the legion Thalassinidea anomobranchiata of Dana. It 

 has been established on the strength of Milne-Edwards's description of Callianidea, and 

 Guerin's description of Iscea (Callianisea, Milne-Edwards; Callisea, Dana), but which 

 (from an examination of specimens lent to me by Dr. Carte of the Dublin Museum) I am 

 inclined to place in the same family as Callianassa. The two genera resemble each 

 other very closely in all points except the formation of the pleopoda. Those of the 

 second pair in Callianassa are biramose ; the inner branch slender, the outer of extreme 



