Contributions to the Crustacean Fauna of South Africa. 379 



TEIBE VALVIFERA. 



This tribe contains two main families, the Astacillidae and 

 Idoteidae, with two other families, the Pseitdidoteidae and Ameso- 

 podidae intermediate in characters between them. The differences 

 between these families consist in the shape of the body, the structure 

 of the anterior peraeopods, the proportional size of the ramus of 

 the uropod and the presence or absence of a second (rudimentary) 

 ramus, the structure of the first pair of pleopods, and a feature to 

 be mentioned below. As regards the first pleopods the Idoteidae 

 have a short peduncle with soft, simple, uuornamented rami ; the 

 Astacillidae have a long- peduncle with the rami frequently transformed 

 in shape and modified, and more or less strongly chitinised. 



Of the intermediate families the Pseudidoteidae combine the Idoteid 

 shape and anterior peraeopods with the Astacillid uropod and first 

 pleopod ; the same is true of the Amesopodidae except that the 

 body-shape is intermediate and the character of the first pleopod is. 

 unknown. 



Turning to the features distinguishing this tribe from all the others 

 I may mention one which has not, so far as I am aware, been insisted 

 on the position of the opening of the vasa deferentia in the male. 

 In Isopods generally (and in Amphipods) the vasa deferentia open on 

 the seventh peraeon segment, either about the middle of the segment 

 or on the posterior margin. 



In the Valvifera, however, the openings have shifted on to the 

 first pleon segment. The two positions can be well seen by comparing 

 a Sphaeromid with an Idoteid. 



Bate and Westwood (Brit. Sess. Crust, vol. 2, pp. 368, 380) quite 

 correctly describe the position of the male-stylets (through which the 

 vasa defereutia open) in Arcturns longicornis and (with figure) Idotea 

 tricuspidata (= baltica). On the other hand G-erstaecker (Bronn's 

 Thierreich, Bd. 5, AM. 2, p. 101) does not mention specially for the 

 Valvifera the position of the openings of the vasa deferentia in the 

 text, but figures them for Idotea (Mesidotea) entomon on the first 

 pleou segment (pi. 4, fig. 12). Sars represents the male-stylets of 

 Idotea baltica as on the seventh peraeon segment (pi. 32). 



In all the examples of Valvifera that I have examined the male- 

 stylets open on the first pleon segment, although I have been guilty of 

 carelessness in this respect in my recent descriptions of South African 

 Valvifera. 



So much for the position of the openings of the vasa defereutia. In 

 the majority of Isopods they do not open flush with the ventral surface 



