(XXXTI)] CRUSTACEA CASPIA. 53 



water forms, which have adapted themselses to living in somewhat bra- 

 kish water. A species of Tlielphiisa is besides found in tlie rivers debouching 

 into the southern part of the Caspian Sea, but this form cannot properly be 

 referred to the fauna of the Caspian Sea itself. Unlike these 3 forms, the 

 Mysid(B are generally regarded as being of true marine origin, though 

 some species are also occasionally met with in pure fresh water lakes, as 

 first stated with regard to the BIysis relicla of Lo ven. But, as indicated by the 

 specific name, this species is believed to be left from a remote time, when 

 the lakes were in connexion with the Ocean, and this view, set forth by 

 Prof. Loven, has been subsequently fully confirmed by a closer comparison 

 with the marine form, Mysis oculata Fabr., abounding in the arctic Ocean. 

 The Mysis relida of Loven is indeed quite certainly a depauperated descend- 

 ent of that species, exhibiting, as it does, a very close resemblance to imma- 

 ture specimens of the former'). A similar descent from true marine forms 

 may in all probability also be attributed to the other Mysidse found to inhabit 

 pure fresh water. 



The Mysidse of the Caspian Sea are as far as is yet known all comprised 

 within the subfamily ilf?/sfH«, as defined by Mr. Czerniavsky, and belong to 

 4difi^erent genera, one of which is now for the first time established. Of the 

 8 species enumerated in the following pages, 4 are as yet not known beyond 

 the Caspian Sea, whereas the other 4 are stated to be common also to the 

 Black Sea. As above stated, most of the Caspian species have already been 

 described by Mr. Czerniavsky in his valuable work, «Monographia Mysi- 

 darum imprimis Imperii Rossici». But, as these species are treated of in con- 

 nexion with species from other parts of the Ocean, it is somewhat difficult 

 at once to get a view of those occurring in the Caspian Sea. As moreover 

 some of the species have been rather imperfectly described, and good habi- 

 tus-figures are not at all given, I have thought it right to re-describe all the 

 species, which I have had myself an opportunity of examining, and to give 

 both habitus- and detail-figures of all. The descriptions are made as short 

 and concise as possible, and are chiefly confined to the real distinctive cha- 

 racteristics, the oral parts being, as a rule, only treated of in one species 

 of every genus, as there are generally no appreciable difi'erences to be found 

 in these parts in species belonging to one and the same genus. Good figures 

 will on the whole make a minutely detailed descrii)tion superfluous. To the 

 descriptions are added short critical remarks on the validity of the species 

 and its relation to other nearly allied species, as also information as to 

 occurrence and distribution. 



1) See G. 0. Sars, Histoire naturelle des Crustacees d'cau douce de Norvcge I. p. 40. 



Melanges biologiques. T. XIII, p. 401. 



