352 PERCY SLADEN TRUST EXPEDITION 



on the dactylopodites of the walking legs of Coralliocaris, of which rudimentary or 

 vestigial stages are shown by the subgenus Onycocaris, and in the horns of the eyes 

 of Corniger, while the subfamily as a whole presents, from genus to genus, a very 

 pretty example of the loss of an organ by gradual stages in the reduction of the 

 arthrobranch of the third maxilliped, from the well-formed gill of Urocaridella to the 

 evanescent lobe of Pontonia. But there are other cases in which an organ is always 

 present in the same degree of perfection. This is perhaps not very remarkable, for 

 instance, in the peculiar armature of the fingers of the chelse of certain Periclimenes, 

 such as the pits of P. petitthouarsi and P. spinifera or the combs of the same and 

 certain other species, where imperfect stages may well have disappeared ; but it is 

 most striking in the many cases of well-formed and characteristic spines on the limbs 

 and body which come and go throughout the subfamily without ever a trace of 

 rudimentary or vestigial stages. Of such facts as these any theory of evolution must 

 take account. 



Finally it is necessary to state that even less conception can be formed of the 

 nature of a "species" in the Pontoniinee than in many other groups of animals. We 

 have not the slightest knowledge as to what degree of fertility exists between the 

 assemblages of individuals to which this name is given, and have not even the assurance 

 that some of them are not merely allelomorphs. The attempt has been made in some 

 instances to recognize certain entities as " varieties," but as none of these are connected 

 by intermediate individuals I have felt bound, in accordance with principles which 

 I have stated elsewhere*, to class them all in the same temporary category as the 

 "species." 



The Species of Pontoniinse. 



In the following lists the species of the subfamily at present known are arranged 

 under their genera with a key to the species of each genus. 



Genus Urocaridella Borradaile, 1915. 



Ann. Mag. Nat. Hist. (8), xv. p. 207. 



Definition : Body slender, much compressed ; sixth abdominal segment elongate ; 

 rostrum long, slender, upcurved, with teeth above and below ; outer flagellum of 

 antennule deeply cleft ; antennal scale long, narrow ; mandible with palp ; second 

 maxilliped with podobranch, and last joint mediad of preceding joint ; third maxilliped 

 slender, with arthrobranch ; legs very slender, directed forwards, with simple, slender, 

 nearly straight dactylopodites ; first abdominal limb with appendix interna. 



Type: Urocaridella gracilis Borradaile, 1915 (Plate 53, fig. 2). 



Ann. Mag. Nat, Hist. (8), xv. p. 210. 



Definition: Rostrum nearly twice as long as carapace, much upcurved, excavate 

 at base, its formula ^^f-^, one of the dorsal teeth standing in the middle of the 



* Gardiner's Fauna of the Maldives, vol. i. 



