Prof. AUman on Chelura terebrans. 



yet wc believe the rest of its organization as well as its habits will 

 separate it as far from the family of the Gammariday as this fa- 

 mily is separated from that of the Hyperidce (famille des Hype* 

 rines, Edw.). 



We believe that in the order Amphipoda an important syste- 

 matic character available in the formation of our higher groups 

 will be found in the condition of the last four rings of the abdo- 

 men with their appendages. In the entire order of Amphipoda, 

 the abdomen (using this word in its ordinary acceptation among 

 carcinologists) is divided into two very distinct regions, an ante- 

 rior and a posterior. The former is composed invariably of three 

 distinct rings, and supports three pairs of isomorphous natatory 

 feet ; the posterior portion also bears three pairs of appendages, 

 but these are never formed like the feet of the anterior region*. 



In the two families of Gammarida and Hyperida, constituting 

 the entire order oi Amphipoda in the system of M.Edwards, the 

 appendages of the fourth and fifth abdominal rings (the first two 

 rings of the posterior region) are similar to each other, and con- 

 sist each of a long cylindrical peduncle terminated by two pieces 

 whose form varies within narrow limits in difierent genera, and 

 the rings which support these appendages are distinct from one 

 another. 



In Chelura however it is very difi"erent ; we are here at once 

 struck with the great size of the posterior region of the abdomen, 

 and we find that the first, second and third ring of this region 

 are consolidated into one great trunk which bears three pairs of 

 heteromorphous appendages. 



From these two very different conditions of the abdomen, 

 therefore, we may obtain characters available in the formation of 

 our primary division of the order. In a zoological point of view 

 I believe we may consider such characters as ranking higher than 

 those which are derived from the conformation of the maxillary 

 feet, a principle indeed which is already acknowledged in the 

 establishment of the order Lcemodipoda, in which the maxillary 

 feet as well as the .other oral appendages are in eveiy respect like 

 those of the GammaridcBy and in which the rudimental condition 

 of the abdomen is almost the sole character from which these 

 Crustacea have been established as a distinct order f. 



* The anterior and posterior regions of the abdomen here alluded to must 

 not be confounded with those to which Erichson, in his more philosophic 

 views of the import of the parts among the Crustacea, has applied the same 

 phraseology ; the anterior region of this author consisting of a large portion 

 of the so-called thorax, while his posterior region is made up of those seg- 

 ments to which the term abdomen is exclusively given by the generality of 

 Avriters, and to which, in order to avoid misconception, we have thought it 

 better to restrict it here. 



t The universal absence of mandibular palps in the Lcemodipoda has been 



