152 CRUSTACEA. 



power of motion. The ocular spots even when flat may generally 

 be recognized with care from the difference in their minerali- 

 zation ; they are often dark-coloured as if retaining some of their 

 pigment, and have a slightly granular aspect, corresponding in 

 fact very closely, both in position on the shell and in apparent 

 structure, with the double-eyed Cypridinia of the Indian Ocean. 

 In one species there is a short sulcus extending on each side 

 from the medial line behind the eye obliquely backwards and 

 outwards, reminding us of the perhaps somewhat similar nuchal 

 furrow of Apus. I only know the genus in the upper Silurian 

 rocks. 



Cer-atiocaris solenoides (M*Coy). 



Sp. Char. Sides meeting along the back at an acute angle, each 

 being nearly four times longer than wide, the ventral margin 

 nearly straight and parallel with the dorsal line ; anterior end 

 narrowed and truncate obliquely forwards and outwards from 

 the dorsal line ; posterior end scarcely narrowed, truncated 

 obliquely backwards and outw^ards ; valves evenly convex, the 

 edge slightly thickened ; ocular spot a little depressed, close to 

 about the middle of the truncated anterior margin ; from the 

 internal (dorsal) anterior angles a small furrow extends a little 

 way obliquely backwards and outwards ; oblique longitudinal 

 striae very close and fine ; eyes two-thirds of a line in diameter ; 

 width of each side from dorsal to opposite margin 5^ lines. 

 This much resembles a little Solen in form. The ocular spot 



is generally dark-coloured. 



Common in the Upper Ludlow rock of Benson Knot. 

 {Col. University of Cambridge.) 



Ceratiocaris ellipticus (M^Coy). 



Sp. Char. Each side longitudinally elliptical, evenly convex, 

 about twice and a half longer than wide, greatest width of the 

 side and curvature of the margin about one-third from the 

 anterior end, which is elliptically pointed ; posterior end ob- 

 tusely rounded, the oblique truncation nearly effaced ; ventral 

 margin convex ; ocular spot elevated like a small tubercle, 

 twice its diameter from the dorsal line, and about one-fourth 

 the length from the anterior end ; I believe the striae of the 

 surface have the direction usual in the genus, but they are 

 very delicate. Length 1 inch 3 lines, greatest width of the 

 sides 6 lines. 

 The elliptical form, prominence of the eye-spot, and its distance 



from the anterior end, mark the species well. 



Rare in the Upper Ludlow rock of Benson Knot. 

 [Col. University of Cambridge.) 



