THE OSTRACODA. 243 



of the difficulty may lie in Glaus having had both viltimate and penultimate stages 

 before him *. 



The differences which Miiller in his text points out between these species are two : — 



(1) Differences in the armature of the principal bristle of antenna i. ; in my specimens 

 the proximal teeth are displaced, generally less so than in his figure of magna (xxi 26), 

 more so than in his description of suharcuata. They are spaced as in his figure of sub- 

 arciiata (xxi. 12) and not shaped quite like those of magna (xxi. 26). Glaus assigns 

 about 20 pairs of teeth to magna, set more closely and more regularly distally than 

 proximally f ; this agrees with my specimens ; but to suharcuata he attributes 30 pairs. 



(2) Differences in the hook of the inner branch of antenna ii. The right hook in my 

 specimens agrees better with magna than with suharcuata in being bent at first sharply 

 through a right angle, and then curved boldly through a second right angle ; on the 

 other hand, it is said by both Muller and Glaus that in suharcuata it is bent at first 

 through an acute angle %. (3) In Claus's figures, the posterior border of the male shell 

 of suharcuata (iii. 3) begins with a concave curve, and projects much further than that 

 of magna (iii. 1). My shells agree best with magna. 



On the whole it seems that the penultimate stage of the ' Research ' specimens must 

 be assigned to magna, Glaus, leaving open the question whether the suharcuata of Glaus 

 and Muller is founded on individual variation of magna, or is a separate species. 



Stage I. {macrocheira stage). 



Female (Mean, 3'36). — Shell very oblong; shoulder-ridge slight; postero-dorsal 

 angle slightly rounded, posterior border a steady curve ; right asymmetrical gland 

 slightly projecting ; ventral border arcuate, the shell about as high in front of the 

 arcuation as behind it ; anterior border bluff, rounded. Frontal organ and Antenna i. 

 as drawn by Muller (xxi. 5, 6), with the addition of a bunch of short spines on the 

 terminal joint of antenna i. at the base of the principal bristle. Dorsal bristle with hmg 

 hairs. Antenna ii. with its inner branch as drawn by Miiller (xxi. 8). No basal 

 bristles. 



Male (Mean, 2'91). — Shell of the same general outline as in the female, except for 

 (1) a more marked shoulder-ridge, (2) astraigliter posterior border, (3) a more forwardly 

 projecting rostrum. Frontal organ as drawn by Miiller (xxi. 7). Antenna i.: the 

 armature covers a little less than a third of the length of the principal bristle ; it amounts 

 to about 29-31 pairs of teeth ; the order in pairs is retained throughout the row ; the 

 first few and last few are shorter and closer together than the rest. Accessory bristles 



* It is even possible that magna and suharcuata arc ono species, allowing for individual variation. 



t The displacement of the teeth from, a regularly paired row into an alternating series is often difficult to deal 

 ■with, since a very slight twist of the axis of the bristle serves to produce this effect : in two antennae of the same 

 specimen, the teeth may appear at first to be greatly displaced proximally in the one, hardly at all in the other. 

 Further, unless the teeth are in exact profile, refraction will sometimes produce the effect of a second tooth behind 

 the first, on focussing downwards. 



X Here again, as with the armature of antenna i., everything depends upon getting the organ into exact profile in 

 one focal plane ; if it be tilted away from the observer, a right angle may easily look like an acute angle. 



SECOND SERIES. — ZOOLOGY, VOL. X. 41 



