382 Records of the Indian Muscnin. [Voi<. XII, 



Group III. 



P. lowisi, sp. nov. i Endopod of last four pairs of pleo- 



P. pUosus, sp. nov. 1 P°^' comparatively well developed 



P. candidus,sp. nov. ) 'l'. ^^.^^ ^^^^^^^^ "! ^^"^^l^^, ^^PP^."" 



P Uebs sp nov . ^^^^^"^^ present on all four pairs 



■^ '■ ■ I in male, but entirely absent in female. 



Group IV. 



Endopod of last four pairs of pleo- 



pods large and well developed in 



P. sabsechota, Kemp <| female, quite rudimentar}" in male. 



Appendix interna absent in both 

 sexes. 



Group V. 



r, 1 ■ .- TT -1 j_ i Endopod of last four pairs of 



P. bisptnosus, Hailstone i . / , ij-u.Lt. 



J IT' 1 J ? pleopods much reduced m both 



and westwood. a j- • ^ 1 



7-, , . ,• TT -1 .>_ i sexes. Aopendix interna absent 



'^ ' m both sexes. 



P. echimilatus (M. Sars), P. vicionensis, Fulton and Grant, P. 

 heiidersoni, Kemp, and P. parvirostris , sp. nov., represented in the 

 Museum collection by examples of one sex only, also in all prob- 

 abilit}^ belong to this group. 



In Group I the appendix interna is usually tipped^ with a series 

 of small coupling hooks, which are ill-developed or absent in other 

 groups that possess this appendage, In Group V the separation of 

 a distinct segment at the base of the endopod is clearh- marked, 

 whereas in Group I the division is obscurely indicated.^ 



P. sabsechota, the sole .species comprised in Group IV, is appa- 

 rently an abnormal form, the distinction between the sexes in the 

 size of the endopod being the reverse of that found in any other 

 species which in this respect exhibits sexual differences. The re- 

 maining groups clearly form a morphological series characterised 

 by the progressive reduction of the endopod and b}' the reduction 

 and suppression of the appendix interna. Group I in which the 

 pleopods have retained their full development is clearly the most 

 primitive, while Group \' in which they are more reduced than in 

 any other is the most specialised. It is noteworthy that Group I 

 includes all the deep-water forms in the collection. 



The facts are not only of interest in the light they throw on 

 the development of the species within the genus; they also, as it 

 appears to me, form a valuable clue to the evolution of the family 

 as a whole, for the other genera can be divided into two sections 

 agreeing, respectively, in the development of their pleopods with 



' Cf. \\ ollcback, Hcygens Museums Aarbog, 1008, no. 12, p. 44, icxl- 



