6 G. O. Sars. 



and subequal; terminal claws comparatively short, without 

 any secondary denticles. Ephippium produced anteriorly to 

 a narrow stripe, egg-ampullæ obliquely disposed. Length, 

 without the spine, from 3 to 4V2 mm. 



Remarks. The present form, first described by 

 Mr. King, is closely allied to the New Zealand species 

 D. Thomsoni, G. O. Sars, though differing in some parti- 

 culars, so that it should more properly be regarded as spe- 

 cifically distinct. As Mr. King observes, it is, however, a very 

 variable species. The crest of the head, from which the 

 species has been named, seems indeed to vary considerably 

 in development, thereby giving the head a somewhat diffe- 

 rent appearance in specimens from different localities, as 

 shown in the figures here given. Mr. King has figured 

 a very extreme variety, in which the crest even forms 

 a strongly curved lamina projecting from the cervical 

 region. The shape of the shell itself would also seem 

 to be subject to some variableness, according to local- 

 ity, perhaps also according to the season. Notwithstand- 

 ing this great variableness, there are to be found certain 

 characters common to all the varieties, by which this spe- 

 cies may be readily distinguished from the New Zealand 

 form. Thus the rostrum is never, as in that form, deflexed, 

 but forms always the immediate continuation, of the ventral 

 edge of the head. The shell-spine in all the individuals I 

 have examined, is considerably longer than in D. Thomsoni, 

 and, as a rule, also more upturned. Moreover, the denticles 

 of the dorsal margin of the shell are continued in front as 

 far as the cervical region, whereas in the New Zealand form 

 they are restricted to the posterior half only of this margin. 

 Finally, the tail is more regularly conical in form, and the 

 terminal claws comparatively much shorter. 



