C(ECUM. 179 



ated. The surface is densely annulated with coarse and 

 regular sulci, that vary as to approximation in different 

 specimens, but for the most part become more crowded 

 towards the aperture. Both these and the intervening 

 ridge-like spaces, which latter often present a somewhat 

 imbricating appearance, for the most part, exhibit under 

 the microscope extremely close and equal-sized longitu- 

 dinal striula^ that appear in some to be almost imper- 

 ceptible, in others to be strongly impressed. The dorsal 

 or arcuated side of the shell is clearly, though not greatly, 

 the longer. The mouth is suborbicular, and a little 

 contracted. 



Brown's figure of siriatus is an excellent representation 

 of the adult form of this species, and corrects his statement 

 that it is destitute of a posterior knob. 



" I have as yet," observes Mr. Clark, " met with only 

 two phases of this shell, the taper produced arcuated 

 form, and the shorter less curved final condition." 



Animal pure white, mantle very thick and fleshy, 

 fitting the shell closely, and not extending beyond its 

 anterior margin ; body elongated and slender, head long 

 and flat, cloven at the extremity and closely ridged sub- 

 transversely, on all occasions in advance of the foot: fissure 

 of the mouth vertical ; tentacula short, rather thick, sub- 

 cylindrical, setose, and slightly clavate at the extremities ; 

 eyes very minute, black, not raised on any eminence, 

 placed nearly in a line with the tentacula, at a short dis- 

 tance from their bases. The neck is furnished with lon- 

 gitudinal ridges, and on each side of its centre there are 

 two frosted, yellowish white, contiguous round lines form- 

 ing a very decided canal or groove, the points of which 

 terminate anteriorly at the immediate base of the eyes, 

 and posteriorly at the furthest end of the neck, on the 



