MYSIDACEA— TATTERSALL. 287 



distribution, but its apparent discontinuity will, no doubt, be explained away by 

 future faunistic work. 



Genus MYSIDETES, Holt & Tattersall, 1906. 

 = Metamysidella, Illig, 1906. 

 11. Mysidetes posthon, Holt & Tattersall. 



Mysidetes posthon, H. & T., 1906, p. 10 ; Tattersall, 1908, p. 33, pi. VII, figs. 1-13 ; Hansen, 

 1913, p. 17, pi. II, figs. 2a-c; Zimmer, 1914, p. 402, taf. XXV, fig. 36; Hansen, 1921, 

 p. 3 ; M. similis, Zimmer, 1914, p. 402, taf. XXV, figs. 37-42 ; ? M. Uligi, Zimmer, 1914, 

 p. 404, taf. XXVI, figs. 47-49. 



Occurrence. — Antarctic. Station 314, two males, three females, 28 mm. ; 

 Station 338, one male, posterior end only ; Station 348, one head, three tails ; 

 Station 351, one immature, 9 mm. 



Remarks. — Hansen (1921) has shown that this species is subject to considerable 

 variation, in consequence of which Zimmer's species, M. similis, cannot be main- 

 tained. With this opinion I am in agreement. Zimmer distinguishes M. similis 

 from M. postJwn by the following characters :— (1) In 31. similis the rostrum is longer 

 than in M. posthon, its margins more strongly upturned, and it partially covers the 

 ej^estalks, whereas in M. posthon the latter are not covered by the rostrum. (2) The 

 upper of the two spines on the outer part of the basal joint from which the scale 

 springs is much shorter than the lower in M. similis, whereas in M. posthon the 

 spines are more or less equal in size. Hansen has already dealt sufficiently with 

 the variation in the shape and form of the rostral plate, and shown that that 

 character is not constant enough in the species to form a basis for separation. The 

 " Terra Nova " specimens are all badly mutilated, and it is impossible to examine 

 critically the form of the rostral plate, but so far as I can make out it is in all cases 

 similar to M. posthon, short and not covering the eyestalks. With regard to the 

 second character Hansen has likewise shown that there is considerable variation. 

 In the present specimens, three conform more or less to the condition described for 

 M. similis and four to M. posthon. 



The specimen from Station 351, which I regard as a young stage of M. posthon, 

 presents one or two interesting features. There is a gap in the armature of the 

 lateral margins of the telson, five or six spines on the proximal widest portion of the 

 telson being separated by an unarmed portion from the distal series. Such a 

 condition has been described by Illig in M. kergiielensis and by Zimmer in ilf . illigi, 

 and considered by these authors as a specific character. I regard it as evidence 

 of immaturity. Traces of such a gap can be seen in much larger specimens, and 

 only in fully grown specimens is the series of lateral spines on the telson apparently 

 uninterrupted. The spines on the telson appear to be formed in two groups, a 

 proximal group of four or five and a distal series which develops from the posterior 

 end and extends anteriorly, increasing in number, with growth. In fully grown 



