REPOET ON THE CRUSTACEA MACRURA. 201 



lar processes of the dactylos is a large cusp or blunt tooth, the inner side of which 

 is hollowed to receive the tubercular articulation of the dactylos. The dactylos reversely 

 corresponds with the pollex, but it is smooth on the outer margin excepting two or 

 three teeth near the distal extremity; the inner surface is bordered by a row of cusps more 

 numerous but less prominent than those on the pollex : the carpos is distally 

 furnished with two strong cusps that correspond with the articulating processes of the 

 propodos on the upper and lower distal margins, on the inner surface there is one sharp 

 tooth near the middle of the anterior margin, two, one very large, on the upper 

 margin, and two small cusps on the under : the meros is broader distally than at the 

 ischial extremity, it is smooth and slightly arcuate longitudinally on the upper surface and 

 convex on the lower, it is armed on the outer margin with a row of teeth that gradually 

 increase in size anteriorly, and with two rows on the inner : the ischium is short, flat, 

 unarmed, and articulates with the meros on the inner and outer distal angles by means 

 of strong cuspdike processes, and at the outer coxal angle is a posteriorly projecting 

 process that affords resistance to the to<?'free backward movement of the joint: the 

 coxa is triangulate and articulates at two points, one externally corresponding with the 

 posterior angle of its own somite, and the other internally with a projecting process on 

 each side of the median ventral carina ; attached to the coxa on the outer surface is 

 a large mastigobranchial plate, which is divided into two longitudinal processes, to the 

 upper or median angle of which a podobranchial plume is attached throughout its 

 entire length. The anterior process of the plate dips down longitudinally between the 

 anterior and posterior arthrobranchiate plumes, its lower margin reaching to the 

 pleurobranehial plume ; the posterior passes over the posterior arthrobranchial plume 

 and separates it from the podobranchial (vide sectional diagram of branchiae in 

 PI. XXVIIL), the long hairs that cover the inner surface pass between the several fila- 

 ments of the plumes and probably prevent them from pressing unduly against each other. 

 This arrangement appears to be constant with all the appendages, excepting that the 

 mastigobranchial plate is less pronounced posteriorly than anteriorly ; the posterior pair 

 carries a pleurobranehial plume only, attached to the posterior somite of the pereion. 



Attached to the membranous articulation, between the coxa and the somite below 

 the arthrobranchial plumes, is a lunate appendage (mt), the homology of which is 

 difficult to interpret ; there are four pairs, one attached to each pair of pereiopoda 

 excepting the posterior. I have not been able to find it in our British species of Astacus, 

 but it exists in the three species of Astacopsis in this collection, and also in Astacoide* 

 raadagascarensis. I have not yet had the opportunity of examining other genera. 



I know of nothing that these parts can be homologous with, unless they be the rudi- 

 ments of the foliaceous appendages forming the incubatory pouch of those females 

 that carry their ova beneath the pereion. It might seem an objection that in Astacopsis 

 they exist in. the males as well as in the females, but if they be rudimentary and 



(ZOOL. CHALL. EXP. — PART LII. — 1886.) Fff 26 



