THE OSTEACODA. 245 



acutely pointed, with a few fine spines ventrally. Sometimes the point is less developed, 

 or the organ ends bluntly *. Antenna i. The j)rincipal bristle is of female character, 

 that is, it carries a few scattered m.inute teeth. The two secondary llagella arc a 

 good deal shorter than the principal and are differentiated from the sense-tubes. 

 Antenna ii. without hooks ; there are (?) no basal bristles ; the longer central bristles 

 carry short hairs as in the female of this stage. 



The two sexes can generally only be distinguished at this stage by the three bristles of 

 antenna i. in the male, and by the presence of the penis (not always easily seen). 



The a Larv^. 

 (Plates 21, 22. figs. 161-168.) 



The morphology of the larvae classed under this head leaves little doubt that the 

 greater niimber of them are attributable to magna. But it became obvious, at an early 

 period of the general investigation, that other forms were possibly represented among 

 them, namely liyalophylliim, zetesios, spinlrostris, and lorlcata, forms belonging to the 

 magna and lorlcata groups t- The specimens of these larvae were very numerous, and 

 the difficulties of determining their length and sex were considerable. It is possible that 

 they might have been disentangled by another six months of work, but even then a 

 satisfactory result was not assured beforehand, nor did the probable gain seem likely to 

 compensate for the labour involved. All the larvae, therefore, which exhibited certain 

 characters have been classed as " A." 



The negative characters were : — the general shape of the shell, in not showing the 

 rounded contours of curta, pusilla, and I'otmidata (species which are eliminated by other 

 tests), or the elongate type of procera, brachyaskos, elegans, spinifera, and inennis, or 

 the elongate rostra of daphnoUles, imbrlcata, and ametra ; and the opening of the right 

 asymmetrical gland, not projecting as in rhyncliena, nor displaced dorsally as in haddoni; 

 in several of these species the larvae have been satisfactorily identified. 



The positive characters were a nearly straight posterior border, a straight or slightly 

 arcuate ventral border, with the greatest height of the shell markedly in front of the 

 middle of the length ; a normal position of the asymmetrical glands, tlie opening of the 

 left one protruding above the dorsal border when viewed from the right side ; the frontal 

 organ and central bristles of the second antenna closely resembling the type found in 

 magna at Stage III. The two latter characters, combined with a general similarity of 

 appearance (which is naturally more striking in the actual specimens than in outline 

 drawings), are the reasons for regarding the typical A larvae as belonging to magna. 

 Their characters may be thus defined : — 



Stage IV. (Mean ± OS). 



The males and females are only distinguishable from one another by the presence or 

 absence of the two rudiments of the penis. Shell with a nearly straight posterior border, 



* Query, another species '? but no other difterenco from tho normal type was detected. 



t It is conceivable that tho members of tho mollis group, namely kampta and tyloda, may hare had epiplanktonic 

 larva; ; but as only two adult specimens of each were taken, they can be left out of count. 



41* 



