I0 CRUSTACEA MALACOSTRACA. 



abdomen are immovably fused, but the lateral, very oblique lines between them are sutures, while on 

 the median part of the dorsal surface only impressions are found. 



The aberrant group Ischnomesini (formerly the genus Ischnosoma] shows very interesting fusion 

 of the segments. In all genera the head is immovably united with the first thoracic segment, and the 

 very curved dorsal line between them is only an impression. While in the genus Ischnomesus the 

 posterior thoracic segments and the abdomen are movable, with real articulations between them, we 

 find in Heteromesus the two posterior segments and abdomen immovably fused, and limited only by 

 dorsal impressions. And the genus Haplomesus goes still further, having the extremely long fifth 

 segment fused with the next, so that almost two-thirds of the body is shaped nearly like a stiff stick, 

 with thickenings and impressions on its posterior part. In the ffaploniscus-group pronounced fusion 

 of segments is also found. 



In 1905 my paper: On the Morphology and Classification of the Asellota-Group 

 of Crustaceans .... (Proc. Zool. Soc. London, 1904, Vol. II) was published. Two main-points in that 

 treatise must be taken up again. It is a well-known fact that the males of this sub-order have five 

 pairs of pleopods, but the females only four pairs, and I pointed out that the three posterior pairs are 

 homologous in the two sexes. Consequently it must be either the first or the second pair which are 

 wanting in the females. As the first existing pair in the female form an undivided operculum in all 

 genera of Asellota excepting Asellus (with Mancasellus Harg. and Coecidothea Pack.), and as in all these 

 genera the first pair in the male have their sympods coalesced in the median line or, in Stenctrium, 

 completely fused, I expressed the opinion that it might be concluded that the first pair were present, 

 the second pair wanting in the female. But later I received from Dr. A. Vire three specimens of a 

 most curious animal, Stenasellus Virei Dollf, in which the abdomen has its two anterior segments 

 well developed, a feature not distinct in Asellus or any marine form of Asellota, but this peculiar 

 structure made it possible to see that the first abdominal segment has in the female no pleopods, 

 while such are found on second segment Stenasellus is more allied to Asellus than to any marine 

 genus of the sub-order. The posterior part of the body of Haplomesus tenuispinis n. sp. seen from 

 below (PI. V, fig. 4 f) shows clearly that the female operculum is not attached to the first abdominal 

 segment. And it must now be considered as certain that the females of the Asellota have no pleopods 

 on first abdominal segment, while their first pair, in most genera fused and constituting an operculum 

 without even any vestige of a suture in the median line, are homologous with second pair in the male. 



It is seen that in 1904 I was led astray by the fact that in the males the first pair of pleopods 

 are coalesced or fused in the median line (excepting in Asellus and other fresh-water forms), while the 

 two male pleopods of second pair are quite independent in all forms examined before 1905 by any 

 author. But in Pseudomunnopsis Beddardi Tatt collected by the "Thor" I have now found pleopods 

 of second pair in the male fused to such a degree that they constitute a large plate without any 

 median suture, and only with a somewhat deep and moderately narrow 'posterior incision (PI. XIV, 

 fig- 3^), an d this plate is quite similar to a female operculum, excepting that it has the incision 

 mentioned, in which the small copulatory organs are found. And in another form, Paramunnopsis 

 oceanica TatL, I found the same pair of male pleopods fused in about one-third of their length (PI. 

 XIII, fig. ni), thus intermediate between Pseudomunnopsis and the great multitude of genera with the 



