1904.] ASELLOTA-GROUP OF CRUSTACEANS. 307 



be appropriate to discuss the structure of the other Asellota, 

 especially their pleopoda. It may be added that, according to 

 descriptions given by S. I. Smith and A. S. Packard, the two 

 genera Mancasellus Harg. and Gceddothea Pack, are closely allied 

 to Aselhis in the structure of antennae and pleopoda, the essential 

 difference being that the endopod of the second male pleopod has 

 no pi-ocess from the basal joint. 



TV. On the Structure of the Asellota, Asellus and 

 Stenetrium excepted. 



Some years ago A. Dollfus described and figured (Bull. Mus. 

 d'Hist. Natur. Paris, 1898, no. 1, p. 37, figs. 2 & 2 a) a very 

 curious animal, Stenasellus virei Dollf . ; unfortunately he had 

 only one minute and mutilated specimen from fresh water in the 

 Oevennes. He refers the genus to the Asellota. His description 

 together with the two figures are certainly sufficient for the 

 recognition of the species, but not for deciding the question of 

 the relationship of the genus. The uropods are as in the Asellota, 

 and the four thoracic legs figured are, so far as can be seen, not 

 very different from those in lanira ; but the animal differs from 

 all Asellota in two features. The author says : " Cephalon intime- 

 ment uni au premier segment pereial," which is not the case in 

 any form of the Asellota hitherto known. The other feature is 

 in the structure of the abdomen. Dollfus writes : " Pleon a trois 

 premier segments tr^s developpes " ; this agrees well with his 

 figures, which show the abdomen as consisting of an oblong 

 " pleotelson " and three segments ; these latter are slightly nar- 

 rower than, and their sum at least half as long as, the posterior 

 undivided portion. But this abdomen differs much from that met 

 with in any of the Asellota or any other group of Isopoda. In 

 Dollf us's description we find as to the pleopoda only the statement 

 that they are " narrow," and they are nearly invisible in his 

 figures ; besides, he does not mention the mouth-parts. Judging 

 from all these circumstances I thought that Stenaselhhs could not 

 be referred to the Asellota, and in the manuscript despatched to 

 London in October I added some further critical remarks. But 

 at the end of November Dr. Armand Vire, the ardent explorer of 

 French caves, most kindly presented me with three specimens of 

 Stenasellus virei captured in August 1904. An examination 

 of these specimens showed that the abdomen has only two free 

 segments in front of the large " pleotelson," and that the animal, 

 in spite of some differences, is rather allied to Asellus in the 

 structure of the mouth-parts and the pleopoda. I commiinicated 

 my conclusions to Dr.Yire, who allowed me to make the necessary 

 corrections in the proof, for which I beg him to accept my sincere 

 thanks. I will therefore state that Stenasellus differs from other 

 Asellota in having the head fused with the first thoracic segment, in 

 having the two anterior abdominal segments well developed, while 

 these are rudimentary in the Asellota (for instance in Stenetrium), 

 and in a few other points. It must, in my opinion, be established 



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