1913] Hovscn : On Some Califdniiaii Xcliizopoda 177 



2. Mysis costata Holmes 



PI. 9. figs. '2a;l 



Of this curious form only a moderately preserved female 

 specimen is at hand. Since Czerniavsky (1882-83) subdivided 

 the genns Mijsis (^sens. Sars 1877 and 1879) into a number of 

 genera, several allied genera have been established by various 

 authors. It is easily to be seen that if most or nearly all of these 

 genera shall be maintained, then 31. costata cannot remain in the 

 genus Mysis together with its type M. oculata 0. Faljr. ; further- 

 more, M. costata can scarcely be referred to any among the more 

 recently established genera, but nevertheless I think it advisable 

 not to erect a new genus for its reception before a more detailed 

 account of the appendages can be given. M. costata is more 

 nearly related to Neomysis Czern. than to any of the other genera. 

 The labrum has a frontal process about as long as in Neomysis 

 vulgaris, but in M. costata this process is very acute and much 

 more slender than in N. vulgaris. The antennal squama has the 

 end narrowly rounded with two or three terminal setae (fig. 2a). 

 The transverse ridges, mentioned by Holmes (1900), on the first 

 to the fifth abdominal segments (fig. 2b) are dorsal foldings of 

 the skin, so formed that, when the animal is seen from the side, 

 the dorsal line of these abdominal segments looks as if a good 

 number of such segments were present with the front margin of 

 each such "segment" covered by the hind margin of the pre- 

 ceding one; but these foldings do not reach the lateral margins 

 of their segments, as of course is the case with the articulations 

 between the real segments. The sixth segment (fig. 26, VI) has 

 the first transverse "ridge" produced into a median, free, tri- 

 angular plate (Holmes naming it a spine is less correct) ; the 

 posterior part of the dorsal surface of this segment is sculptured 

 in a peculiar way, and the middle of the posterior margin is 

 produced in a smaller, triangular plate. 



The telson (figs. 2c-2d) has the proximal third of each lateral 

 margin considerably more concave than in Neomysis, while its 

 outline otherwise is more like that in the last named genus than 

 in any other form of Mysis sens. lat. Bach lateral margin has 

 from the base to near the end fourteen or fifteen long spines; 



