32 REPORT — 1855. 



process, as in Anonyx ampulla (Kroy.); but in tlie more common forms it 

 appears as a plate across the anterior portion, as if it gave strength and so- 

 lidity to the structure. As before observed, this is the sternal aspect of the 

 mandibular segment, and acts as a fulcrum to the labium and anterior 

 portion of the mandible. 



The labium is divided into two parts, the upper and the lower. The line 

 of separation appears to be an imperfect hinge enabling the lower portion 

 (E, fig. 2, PL XII.) to possess a slight opening and closing power, which co- 

 operates with the mandibles in collecting materials into the moutli. 



The margin of the labium is generally fringed with hairs. In Gammarus 

 gracilis many of these are club-shaped and cumbersome in their appearance.. 



The mandibles are powerful organs which impinge at their extremities 

 one against the other, the biting edge being in the median line, and deve- 

 loped into a series of denticles or teeth-like processes (PI. XIV. fig. 6 ft); 

 these vary in form, in some considerably, and perhaps less remarkably in all 

 genera. Within the denticulated extremity a second process commonly 

 exists (PI. XIV. fig. 6 c), like a repetition of the first. It appears not to be 

 always present ; but when it is, the plate is articulated by a free joint with 

 the mandible, and is capable of a certain amount of movement. Situated 

 about the centre of the posterior margin stands a large projection, which 

 meets a fellow in the opposite mandible, and is evidently adapted for 

 mastication ; it may with propriety be called the molar tubercle (PI. XIV. 

 fig. 6 a). It forms with the anterior denticulated edge the t\vo extremities 

 or horns of a crescent. The second or articulated process is placed between 

 the two, but nearer to the anterior teeth. This intermediate plate appears 

 to be constructed so as to pass the food from one to the other, from the 

 biting to the grinding surfaces, between which there are curved spines (rf) 

 to facilitate the movement. 



The two mandibles are brought into contact by powerful muscles, which 

 are attached to the inner surface of the dorsal portion of the cephalic ring, 

 and homologize with those attached to the long calcareous tendons in Ma- 

 croura, which have their muscles secured to the inside of the carapace. 



The surface of the molar tubercle is covered over M'ith rows of teeth-like 

 processes, so minute that they can only be defined by a quarter-inch power 

 object-glass. The arrangement of tiicse teeth is tolerably constant, being 

 in rows more or less even. At the lower portion the teeth are larger, the 

 outer row being most conspicuous ; the size diminishing, row after row, until 

 towards the higher limits, their importance has so fallen away, that they can 

 with great diflficulty be distinguished at all. In some species there is added 

 a filamentary appendage to this tubercle, the margin of which is ciliated 

 with minute hairs. Perhaps this may be in some way connected with taste. 



The. mandibles are no exception to the general law among the Articulata, 

 that all the appendages are modified legs ; the mandible itself homologizing 

 with the ischium or third joint of the perambulating leg, and the same in the 

 gnathopodite of the recent acute but cumbersome homological nomenclature 

 of Prof. Milne-Edwards, the maxilliped of authors generally. 



That the third joint is the correct homologue, unless the second be fused 

 in common with it, we think can be demonstrated by the fact, that in the 

 Macroura the ischium of the third gnathopod (maxilliped) has the inner 

 margin furnished with teeth which impinge against the similarly denticulated 

 edge of the corresponding member, and assumes the character of a not very 

 imperfect biting apparatus. 



In the mandible of the Amphipocla the parts are developed into an 

 efficient and powerful organ ; the denticulated margin has the teeth more 



