CRUSTACEA MAI,ACOSTRACA. IV. 



reach that end. Telson with the proximal part of the lateral margins more convex, while these margins are 

 about at their middle more concave than in P. typicus, according to Sars' figure; the end with 3 spines, but a 

 smaller spine is in both specimens observed on one of the coarsely serrated lateral margins near the end. 



Length of the young male 5.6 mm, of the female with the marsupium rudimentary 5.7 mm. 



Remarks. By the three keels on the carapace and above all by the curious shape of the narrow 

 first free thoracic segment this species differs strongly from P. tvpicus. 



Occurrence. Not taken by the "Ingolf", but by the "Thor" at a single station. 



South of Iceland: Lat. 62°57' N., Long. I9°58' W., 508 fath. ; 2 immature, mutilated specimens. 



Family Diastylidae. 



This large family, which is distributed all over the world, is especially represented in the arctic- 

 subarctic and boreal regions by a large number of species; from the "Ingolf" area 19 species are dealt with 

 in the present paper. But some questions as to the limitation of its genera are difficult ; certain features fre- 

 quently used are of slight value as generic characters. For instance, rudimentary exopods in third and fourth 

 pairs of legs in the female are found in Leptostylis and wanting in Diastyloid.es, while in Diastvlis they are 

 wanting in most species, but according to Caiman (1912) distinct in some forms. Whether third and fourth 

 free thoracic segments are fused or separated is another feature used as generic character, but it has scarcely 

 been observed that every transition between complete fusion of these segments without vestige of dorsal 

 suture and movable articulation of these segments against one another can be pointed out ; a few facts may 

 be mentioned. Sars saj's that in Diastvlopsis resima Kr. (referred here to Brachydiastylis Stebb.) these two 

 segments "are very firmly connected", but "a well-marked suture may in reality be proved to exist between 

 them"; in Makrocylindrus spiniventris n. sp., the suture is distinct on the sides but its transverse part on 

 the dorsal surface is wanting. In the female of the gigantic DiastylisGoodsiri Bell these segments are seemingly 

 both dorsally and on the sides well separated, but a closer inspection shows that the furrow between them 

 is firmlv chitinized, so that they are immovably coalesced. In Diastvlis polaris G. O. S. and Diastvlis longi- 

 caudata Bonnier there is a narrow but real, movable articulation between the two segments, especially on the 

 dorsal side; D. scorpioides Lep. is intermediate between D. Goodsiri and /). polaris; in Diastyloides serrata 

 G. O. S. and Leptostylis grandis n. sp. the articulation is well developed. 



Taking these and other difficulties into consideration, I decided as to the genera to follow a middle 

 way between Stebbing and Zimmer. The genus Adiastylis Stebb. is not adopted, but Brachydiastylis Stebb. 

 is maintained, especially because a new species allied to R. resima Kr. shows some of the peculiarities found 

 in that Kroyerian species developed to a still higher degree, so that these two species constitute a natural 

 group. Finally it may be stated here that for reasons to be given later on a new species is referred to Makro- 

 cylindrus Stebb., though it differs from the diagnosis of this genus in a special feature of the telson. — Five 

 genera are represented in the material. 



