BORRADAILE— ON THE PONTONIIN^ 349 



of the second maxilliped and the greater reduction of that of the third maxilliped. In 

 other respects there is no difference between these closely related genera. While, 

 however, Palmmonella does not yet appear to have given rise to any daughter genera, 

 Periclimenes represents a central stock from which groups of genera diverge in several 

 directions. Periclimenceus is an interesting modification of this stock in which some 

 of the features of Alpheus are reproduced. Pontoniopsis represents a second departure 

 from the same stock. Anchistus, Pontonia and Conchodytes form a third group, charac- 

 terized by degeneration in connection with the habit of living in the mantle cavity 

 of bivalves and ascidians. The species described by Pesta as Marygrande mirabilis, 

 which is however an Anchistus, shows the earliest stage in this degeneration. Its body 

 is still fairly slender and compressed, but the downcurved, toothless rostrum, the stout 

 legs, the great chela, and the broad joint of the third maxilliped betray the tendency 

 which is increased in other species of Anchistus and leads eventually through Pontonia 

 to Conchodytes, where it culminates in C. meleagrinoB. Some of the species of Anchistus 

 retain, in the form of the long joint of the third maxilliped and in a minute dentation 

 of the rostrum, primitive features lost by A. mirabilis. It will be recalled that Anchistus 

 has a vestige of the gill of the third maxilliped which is lost in Pontonia and Concho- 

 dytes, and that a peculiar breadth and hairiness of the laciniae of the maxillule are also 

 less marked in the former than in the latter two of these genera. Possibly the point 

 of origin of this group was somewhere in the neighbourhood of the coral-haunting 

 Periclimenes aurantiacus, which has a toothless rostrum. Pontonides seems to have 

 arisen near this point. A fourth group of forms probably derived from Periclimenes 

 is presented by Harpiliopsis, Harpilius, Coralliocaris, Coutierea, and Stegopontonia. 

 These are sluggish, generally coral-haunting forms, but not internal commensals. In 

 them the legs are stout, with hooked dactylopodites, but the body, though more 

 depressed than in the Pontonia group, is less swollen and degenerate. The rostrum 

 loses in depth, but is straight and nearly always dentate, and the maxillules are of 

 quite a different type. In Harpiliopsis and Coralliocaris the gill of the third maxilliped 

 is comparatively well developed, but in Harpilius it is lost, according to M. Sollaud. 

 The members of this group diverge more than those of the second group, but it is not 

 necessary here to recapitulate their differences, which are given in the key above. 

 Harpiliopsis is the most primitive of them, and it is quite impossible to reconcile 

 the evidence either of its bodily habitus or of its maxillule with any theory of a descent 

 common to it and Anchistus. Possibly it may have taken origin somewhere in the 

 neighbourhood of Ancyclocaris, but it seems more likely to have arisen from Periclimenes. 

 Harpilius, though outwardly it presents no very remarkable feature, is, in its loss 

 of a gill and in the peculiar form of its second maxilliped, perhaps the most aberrant 

 of all. Coralliocaris shows in its third maxilliped, its second cheliped, and the dactylo- 

 podites of its walking legs, certain convergences with Conchodytes. 



The affinities of Typton are doubtful. Its supraorbital spines* and narrow third 



* It is possible that these spines are not present in T. bouvieri. Nobili states in his preliminary 

 description of the species that it has " ocular " spines, and in a later account that antennal spines are 

 present, but in his figure he shows neither. 



