240 Records of the Indian Museum. [Vol. XXIV, 



cavity of lamellibranch molluscs, those of Pontonia in a similar 

 situation or in the branchial sac of ascidians. In both genera the 

 prawns probably enter their hosts when larvae and never leave them 

 throughout the whole period of their lives {vide p. 117); 



The structural changes which they have undergone in response 

 to this remarkable environment are not great. The species are 

 more or less depressed in habit of body and, except for the occa- 

 sional presence of the antennal, all the spines of the carapace have 

 disappeared ; the second legs are very heavy, frequently unequal, 

 and without spines on the ischium, merus or carpus To each of 

 these characters a parallel can be found in other genera of the 

 family. The only structural feature of unequivocal value is afford- 

 ed by the inner lacinia of the maxillula, which is very broad and 

 densely covered with hair. 1 In this respect Anchistus and Pontonia 

 agree with Conchodytes— which also lives in lamellibranchs— and 

 differ from all other genera of Pontoniinae in which the maxillula 

 has been described. 



The characters available for separating Anchistus from Pon- 

 tonia are very slight, though there can be little doubt that the 

 genera constitute two natural groups of species. In Anchistus the 

 rostrum is laterally compressed in its distal half and frequently bears 

 small teeth at or near the apex. The two distal segments of the 

 third maxilliped 2 are always slender and are not twisted as in the 

 related genus. The dactylus of the last three legs is either sim- 

 ple and strongly hooked, or is scoop-shaped with the distal part 

 of the anterior border bent inwards, and with an accessory tooth. 

 Minor distinctions are to be found in the last abdominal somite 

 and telson. The former is bluntly produced on either side of the 

 telson and with the postero-lateral corners more or less rounded, 

 whereas these four angles are sharply acute or spinous in Pontonia. 

 In Anchistus the dorsal spines of the telson are very small and in- 

 conspicuous, in Pontonia they are usually large. 



The distal endite of the maxilla, as in some species of Pon- 

 tonia and most Periclimenes is divided into two lobes. 



Borradaile recognises five species of this genus and also in- 

 cludes, though with some doubt, Milne-Edwards' Pontonia armata. 

 This species can never be identified with certainty from the brief 

 description which has been published, and the same remark also 

 applies to A. spinulijerus (Miers). Pesta's Marygrande mirabilis 

 is no doubt an Anchistus; but the author seems to have confused 

 two distinct species in drawing up his specific description. 



I have myself seen four species of Anchistus, two of which 

 appear to be undescribed. They are distinguished by the following 

 characters : — 



A. Rostrum toothless ; antepenultimate segment of third 

 maxilliped very broad, contrasting strongly in width with 

 twodistal segments; chela of first leg with its lateral edges 



1 SeeBorradnile'k figs. 25c and 26e loc. cil., 191 7. 



2 Borradaile distinguishes Aucliistus from Pontonia by the slenderness of 

 these two segments ; they are, however, equally slender in some species of Pontonia. 



