ON OUK PRESENT KNOWLEDGE OF THE CRUSTACEA. 37 



that is increased in the Anomura, and carried to such a degree in tlie 

 Erachyura, that in some, as in Meiiethceus, Leptopodus, Maia, &c., the first 

 two or three articulations arc not to be distinguished from the surrounding 

 structure except bj- the position of the olfactory opening. 



In the Cancerida3 all the joints of the peduncle (PI. II. fig. 17) are fused 

 together and are so closely implanted in the structure of the facial portion of 

 the two first somites that they assist m.ore or less perfectly in forming the 

 walls of the ocular orbit, the several variations of which are made use of by 

 Alphonse Milne-Edwards as a means of assisting him to distinguish the several 

 genera of the Cancerides from each other, and which, from theii- easily acces- 

 sible position, might be found a convenient aid in assisting to determine 

 genera among fossil forms. 



Among the Amphipoda all the several articulations are distinct from one 

 another and from the body of the animal, and the olfactory organ is carried 

 in a long tooth-like process that is open at the extremity. This arrangement 

 is not so distinct in the Isopoda and the terrestrial Amphipoda. It also dis- 

 appears in certain abnormal forms of aberrant and parasitic Isopoda. 



The next succeeding, or fourth pair of appendages is among the most con- 

 stant in the subkingdom. Within certain limits the mandibles vary with every 

 genus, and would form when detached a very certain means of generic diag- 

 nosis. In the most simple condition, where they approximate in form to that 

 of the peduncular portion of the second pair of antennas, they exist in Nehalia 

 (PL III. fig. 19). But, as stated by Milne-Edwards (" Squelette tegumentaire 

 des Crustace's de'capodes," p. 256, Ann. des Sc. Nat. 1854), the mandibles are 

 not appendages simply applied against the mouth, but occupy of themselves 

 a special cavity, flanking on either side the entrance to the alimentary canal, 

 which, when the two are brought into juxtaposition in the median line, they 

 generally close. The mandible in Nehalia (PI. III. fig. 19) is formed of a long 

 osseous process that projects internally, and is secured by muscular attachments 

 to the internal dorsal surface of the carapace ; a large obtuse-pointed process 

 is projected inwards across the mouth, and antagonizes with a corresponding 

 process on the one opposite. This process is very liable to vary in form in 

 different genera. Beyond this process, at the root of it, springs a cylindrical 

 osseous continuation, at the apex of which are articulated two equally long 

 and important joints. These two joints are homologically the same that form 

 the small appendicular appendage attached to the mandible of all Crustacea 

 (PI. III. fig. 21 ) so persistently that their absence is a fact to be recorded in the 

 structure of special genera, such as Talitrus and Orchestia among the Amphi- 

 poda. In a scientific point of view, this appendage must be part of the 

 primary portion of the theoretical limb. This idea also receives confirmation 

 in the form of the mandibles of the genus Pontia of Milne-Edwards, where may 

 be observed a secondary ramus attached to the extremity of the first joint of 

 the appendicular branch (PL III. fig. 20). 



This appendage M. Mdne-Edwards, in the nomenclature that he has 

 given, proposes to name the protognutli ; but the first joint, or true mandi- 

 bular portion, he calls the proto-coxor/nathite, and the second joint the proto- 

 lasognathite, and the other joints in succession after the names of the 

 respective joints in the ideal appendage which they homologically re- 

 present. While wishing to give all honour to that distinguished carcino- 

 logist for the care and exactitude in determining the several parts of the 

 structure of a crustacean by means of a distinct nomenclature, it is with 

 regret that I am compelled to admit that they would be more practically 

 useful, and consequeutly more generally adopted, if the terms were less 



