CRUSTACEA MALACOSTRACA. III. 



55 



and conspicuously longer than second and seventh pairs; the coxa without any process, seventh joint 

 thin, claw short, frequently scarcely marked off. — Uropods inserted on the margin, at most moderately 

 long, frequently reduced, small. 



Remarks. This group is difficult to arrange among the other types of the family, but I 

 insert it here as being the last of the groups with the molar process belonging to the lamra-type. The 

 group is not only deviating from other Parasellidae, but its genera show most interesting differences. 

 As all hitherto known species, excepting one, have been somewhat imperfectly studied, it may be 

 useful to give a more detailed account. 



In 1866 G. O. Sars established the genus Ischnosoma on a very peculiar form, of which he later 

 gave a full representation in the Account. Later another species was established by Sars, a third 

 species by Tattersall, and four species from the "Challenger" by Beddard. In 1908 Harriet Richardson 

 pointed out that the excellent generic name was preoccupied (in 1829 and in 1832), and instead of it 

 was proposed Ischnomesus Rich. Furthermore the authoress distributed the species hitherto referred 

 to Ischnosoma into four genera, thus establishing three new genera, and described two new species. 

 Having a comparatively enormous material of the present group, viz. Ischnomesus bispii/osus G. O. Sars 

 from Skager Rak and n species, 10 of which new, from the ''IngolP area, I have attempted to find 

 valid characters for the genera and new characters for the species. In reality the genera proposed by 

 Richardson — with the possible exception of Rhabdomcsits based on two very imperfectly known 

 species from the "Challenger" - must be accepted as natural, but the majority of the generic char- 

 acters pointed out by Richardson in the analytical key are wrong or of no value, while other charac- 

 ters can be pointed out. 



To take a few instances. Harr. Richardson said in the key that the abdomen in Ischnomesus 

 bispinosus Sars consists of a single segment; the authoress had not seen this species but had probably taken 

 the character from Sars' figures, which in this respect are not distinct, in reality not accurate. According 

 to my examination of a good material of /. bispinosus the abdomen consists of two segments, the first 

 short but well defined in front and behind by articulations, consequently both segments movable. — 

 Furthermore the authoress said that the genus Haplomesits, based on Ischnosonm quadrispinosum 

 G. O. Sars, has the "abdomen composed of a single segment", while in Heteromesus, to which five 

 species were referred, it consists "of two segments". These statements are misleading. Later I redescribe 

 Haplonicsits quadrispinosus and establish four new species in the same genus; of the five species referred 

 by the American authoress to Heteromesus I have examined co-types of the two species established 

 in her paper, and describe in the sequel four new species. And based on such rich material it can be 

 said, that both in Haplouiesus and Heteromesus the abdomen is both above and below immovably fused 

 with seventh thoracic segment, so that only a more or less pronounced transverse furrow or impression 

 is seen between thorax and abdomen, while a furrow between an anterior short and a posterior large 

 abdominal segment is most frequently wanting on the major part of the dorsal surface, but is distinct 

 towards the lateral margins. But more can be said. In Haplomesus the three posterior thoracic seg- 

 ments and abdomen are immovably coalesced, forming a single piece; in Heteromesus the articulation 

 between the fifth and the sixth thoracic segment is very distinct, while sixth and seventh segments 

 and abdomen are immovably fused; in Ischnomesus not only has the abdomen two movable seg- 



