76 THE VOYAGE OF H.M.S. CHALLENGER. 



general form of this species is very familiar ; many might be named which apjDroach it 

 rather closely, but no described species seems to be absolutely identical with it. The 

 nearest, perhaps, are Cythere horealis, Brady, — an Arctic form, — and Cythere cedichilus, 

 Brady, a fossil of the Antwerp Crag. 



[PI. XIII. fig. 5, a-h. a Carapace of female seen from side, h from above, c from 

 below, d from front ; figures e-h represent simdar views of the male shell. All magnified 

 60 diameters.] 



25. Cythere securifer, n. sp. (PI. XIII. fig. 4, a-li). 



SheU of the female, seen from the side, nearly rhomboidal, highest in the middle, 

 scarcely higher in front than behind, height equal to two-thirds of the length ; anterior 

 extremity obliquely rounded, posterior subtruncated, produced and obscurely angular in 

 the middle ; dorsal margin gently arched, ending behind in an obtuse angle, ventral 

 slightly sinuated both in front and behind, and prominent in the middle ; seen from 

 above, boat-shaped, twice as long as broad, broadest a little behind the middle, sides 

 gently curved, extremities truncated, with strong mucronate projections in the middle 

 line, the anterior narrower than the posterior ; end view quadrilateral, all the sides slightly 

 convex. Surface of the shell coarsely sculptured with deep angular excavations. The 

 shell of the male, besides being more slender than that of the female, has the ventral 

 margin of each valve produced downwards, near the middle, so as to form a conspicuous 

 triangular or hatchet-shaped protuberance, that of the left valve being commonly larger 

 than on the right ; the ventral margins in the female are also produced, but not in so 

 marked a manner. Length, l-50th of an inch {'5 mm.). 



A good series of this remarkable Cythere was obtained oft' Prince Edward's Island, in 

 a depth of 50 to 150 fathoms. (Near Station 145.) 



[PL XIII. fig. 4, a-h. a Shell of female seen- from left side, h from above, c from 

 below, d from front ; figures e-h represent similar views of the male shell. All magnified 

 60 diameters.] 



26. Cythere i7npluta, n. sp. (PI. XVI. fig. 3, a-d, and PL XXVI. fig. 6, a~d). 

 Shell, seen from the side, subovate or subquadrangular, higher in front than behind ; 



anterior extremity wide and obliquely rounded, posteiior narrow, subtruncated or 

 rounded, sometimes produced below the middle, and dentated ; dorsal margin gibbous 

 over the anterior hinge, thence sloping steeply backwards, and ending either in an obtuse 

 angle or a rounded curve, ventral margin convex ; greatest height situated at the 

 anterior hinge, and equal to more than half the length ; seen from above, regularly 

 ovate, widest near the middle, more than twice as long as broad, sides gently convex, 

 or slightly sinuated in the middle, and converging evenly to the extremities which are 

 equal and subacuminate ; end view subtriangular, higher than broad, sides convex, angles 



