274 



NiLEUS SCRUTATOR. (N. Sp.) 



Pig. 259. 



Fig. 260. 



Fig. 261. 



Fig. 259. — Nileus n\acroj)s. 



2G0. — N. scrutator. Upper and side views of the head. 



261. — N. affinis. a, head of a coiled up specimen ; b, under side of the same 

 shewing the pygidium and the thick fold or doublour under the front 

 part of the head. 



Description. — Head nearly semicircular, rather strongly convex, a little 

 less than a i(uarter of a sphere, posterior angles narrowly rounded, length 

 about half the width. Eyes moderately large, crescentiform ; their length 

 about three-eighths the whole length of the head ; their posterior corners 

 situated about half their own length from the posterior margin; the dis- 

 tance between them, measured fi-^m the most projecting point of the 

 outer curve at the base of the visual surface of each, one-eighth greater 

 than the length of the head (the latter measured along the curve) ; the 

 height of the visual surface about one-third the length of the whole 

 eye ; distance from the posterior angle of the cheek to the eye a little 

 greater than half the length of the head. When perfect the visual 

 surface of the eye is closely covered with minute lenses or tubercles not 

 visible to the naked eye, but which can be distinctly seen with an ordinary 

 pocket magnifier. The eye-lid is depressed convex. The anterior cor- 

 ners of the eyes are a little closer to each other than are the posterior. 

 The facial suture departing from the anterior corner of the eye at first 

 curves slightly outwards and then inwards, and runs all round close to the 

 front margin, but does not cut it. Behind the eye it runs outwards, and 

 cuts the posterior margin at a point half way from the eye to the posterior 

 angle of the head. 



Pygidium semi-oval, anterior angles rounded, all pressed convex, not 

 tri-lolied, a shallow concave depression all round close to the sides and 

 posterior margin ; length equal to half its width. Surface smooth. 



The best preserved head is nine lines in length and about seventeen in 

 width ; length of the eye about three and a half lines ; distance of poste- 

 rior angle of the eye from the margin two lines. 



