174 BULLETIN 2 01, UNITED STATEIS NATIONAL MUSlEUM 



1879a) will show that in M. oculata the cleft is much deeper than 

 in M. relicta and there are four spines on the lateral margins of the 

 telson of M. oculata behind the level of the base of the cleft, whereas in 

 M. relicta the last spine of the lateral margin is at the level of the base 

 of the cleft, and, between it and the terminal spine at the apex there is 

 a considerable unarmed area. 



It is generally accepted that M. relicta has been derived from M. 

 oculata and has become adapted to life in waters of low salinity or to 

 fresh water. It may be suggested, therefore, that M. stenolepis has 

 been derived from M. mixta and become adapted to life in shallow 

 water among weeds near land. M. stenolepis is a much more strictly 

 littoral species than M. mixta and Smith has suggested that the more 

 intense distribution of its chromatophores can be correlated with its 

 association with zostera and algae. 



These two cases provide a striking example of parallelism in evolu- 

 tion and one is tempted to speculate on the factors which have been at 

 work in producing these more or less identical results. A lowering of 

 the salinity of the water may be one factor for it is common to the two 

 cases. M. relicta has actually penetrated to fresh water and M. steno- 

 lepis inhabits an area of the coast line in which the water must have an 

 average salinity that is lower than that of the deeper water outside in 

 which M. mixta lives. Salinity, however, cannot be the whole story for 

 M. mixta penetrates the Baltic Sea, and I am not aware that anyone 

 has observed or noted any indications of parallel changes in M. mixta 

 living there. Other factors possibly involved include depth of water, 

 intensity of light, amount of water disturbance and quantity of food, 

 some or all of which may have been contributing factors. The whole 

 forms a natural experiment on such a grand scale that it would be 

 hopeless to repeat or to analyze it in the laboratory. 



Genus PARAMYSIS Czerniavsky 1882 

 Paramysis Czerniavsky, 1882a, pp. 59, 65 ; 1882b, p. 55. 



PARAMYSIS ORNATA (G. O. Sars) 



Mysis ornata G. O. Sabs, 1864, p. 242. 

 Synmysis ornata Czerniavsky, 1882b, p. 27. 



Occurrence. — Kiel* (identified by Mobius) ; Seaham, Northumber- 

 land, England* (identified by A. M. Norman); Helgoland*; Ply- 

 mouth* (identified by G. E. Bullen). 



PARAMYSIS SPIRITUS (Norman) 



Mysis spiritus Norman, 1860, p. 431, pi. 8, figs. 1, la. 

 Synmysis spiritus Czerniavsky, 1882b, p. 28. 



Occurrence. — Helgoland*; Plymouth* (identified by G. E. Bullen). 

 Distribution. — Atlantic coasts of southern Europe. 



