of the North Atlantic and of North- Western Europe. 737 



beak situated about tlie middle, its termination narrow, truncate. Seen from 

 above, ovate, slightly constricted in front, where the valves meet. At the 

 posterior end the middle is bluntly mucronate, and the sides are produced to an 

 acute angle, so as to impart to it a somewhat tridentate appearance ; dorsal ridge 

 prominent, where it bends downwards in front. Surface sculptured with flexuous 

 longitudinal riblets, crossed by a few indistinct ones arranged irregularly. 

 Length, "5 mm. ; breadth, three-fifths the length ; height, fully one-third the 

 length. 



" Habitat. — Off St. Monaco, in 12 to 14 fathoms ; bottom sand and gravel rare. 



" This species somewhat resembles Cytherura acuticostata, but differs in being 

 not so stout, and in having the valves produced backwards, so that the posterior 

 extremity of the shell has a tridentate form." 



We figure Mr. Scott's type specimen, which he has kindly sent to us for that 

 purpose. We believe that it must be regarded as the young of Ci/theriira acuti- 

 costata. We have not a series down to the very young of that species ; but in the 

 young of the genus Cytherura the beak occupies a much larger proportion of the 

 shell, and is thus more prominently conspicuous than in the adult. We have 

 especially noticed this to be the case in C. cornuta and C. nigrescens ; and a very 

 fine series of the young of the former species shows that the lateral projections 

 are also more acute in the young than in the adults. We figure the young of 

 the two last-mentioned species in illustration of our remarks (PL LXiii., figs. 20, 21.) 



Cytherura nigrescens (Baird). 

 Distribution. — East Finmark, tidemarks (A. M. N.). 



Cytherura rudis, G. S. Brady. 



Distribution. — Entrance of Vadso Harbour, in 15—25 fathoms; new to Europe. 



(A. M. N.). 



Cytherura cellulosa (Norman). 

 Distribution. — Vadso, 20—30 fathoms. 



Cytheropteron latissimum (Norman). 

 Distribution. — East Finmark, in 0—25 fathoms (A. M. N.). 



Cytheropteron nodosum, Gr. S. Brady. 

 Distribution. — A single specimen, Vadso, 15—25 fathoms (A. M. N). 



Bythocythere constricta, G. 0. Sars. 

 Distribution, — Vadso, tidemarks (A. M. N.). 



