REPORT ON THE CRUSTACEA MACRURA. XXXV 



or in its absence altogether, even in the adult stage ; in every case where it is present it is 

 reduced in size or rudimentary. 



In Pahvmon, Pandalus, Nauticaris, and Heterocarpus it is three-jointed ; in 

 Paralpheus it is one-jointed ; in Alpheus, Synalpheus, and Spirontocaris it is two-jointed ; 

 in Hippolyte it is wanting, as it is also in Crangon, Nika, Gnathoptylus, Lysmata, 

 Pontonia, Atya, Caridina, Ephyra ' (de Haan not Eoux), and Pasiphsea. 



The psalistoma is very variable in form in different species or genera. In its 

 most characteristic form it appears as a large concavo-convex blade, with a more or 

 less serrate margin which is generally rounded. In some genera the shape of this 

 part is modified ; in others it is diminished more or less conspicuously in size ; while in 

 others it disappears altogether. The greatest variation exists perhaps in the Phyllo- 

 branchiata, and it is rarely present when the synaphipod is absent. The molar process, 

 on the other hand, exists, I believe, universally throughout the higher Crustacea, as a well- 

 developed organ, and it may be seen in the genus Crangon and some near allies without 

 connection with the psabstoma or synaphipod, which generally form parts of the normal 

 mandibles of the Macrura (PI. LXXXVI. fig. id). 



In Crangon and most of the Phyllobranchiata the mandibles are deeply inserted 

 within the oral aperture, whereas in the Trichobranchiata, more especially in the 

 Palinuridse, they are placed at the entrance of the oral tract 

 so superficially that the outer surface of the apophysis is exposed 

 and frequently matted with short hairs. 



Tlie First Siagnopoda. — The first pair of siagnopoda, fre- 

 quently known as the first pair of maxillae, is perhaps amongst 

 the most unchanging of the appendages. It consists generally 

 of three branches, which are always small and of great tenuity; 

 two of the branches, are directed inwards, these are broad and 

 spoon-shaped, and have the inner margins fringed with hairs, 

 more or less densely packed, these bans often increase in strength 



without gaining in length, assuming a smooth and spine-like condition. The third 

 branch is very thin, and is the one which varies most in form, but only within narrow 

 limits ; sometimes it is two-jointed, never more ; it is always directed outwards, and is 

 seldom furnished with more than one or two hairs, which, however, are frequently long. 



Among the Scyllaridse the first siagnopod has never more than two branches, and these 

 correspond to one another and are directed inwards, both being tipped with short spines 

 or hairs. 



This pair of appendages lies close against the mandibles, hugging them on the outer 

 side of the metastomata ; and they appear to be useful in preventing the escape of food 

 from the lateral angles of the mouth, and to be of little use for any other purpose. 



