168 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



finally terminating in a number of loose hair-like fibres hanging 

 free, or looking as if they hung free in the substance of the shell. 

 The same remarks which have been made on Trigonia lamarckii, 

 Gray, apply generally to this shell. It has a purple nacre, shot with 

 indigo shading off into green. One peculiarity of this shell is that 

 the whole inner lining down to the very edges of the valve is 

 closely covered with minute perforations out of which those hair- 

 like nerve-fibres pass into the soft parts of the animal. Nothing 

 of the kind is observed in Trigonia lamarckii, Gray, except a few 

 foramina in the scars for the adductor muscle, and even these are 

 very difficult to perceive ; whereas in the other species, they are 

 quite conspicuous. It becomes very difficult to account for the 

 perforations near the edge of the shell, for one would think that 

 there could be no permanent attachment between the nerve inside 

 the shell and the mantle outside it. However the most remarkable 

 peculiarity in this species is, that in some valves the interior has 

 a few large eyes in pits or depressions, with a regular lens and 

 pupil more like the eyes of Chiton than those of Trigonia. The 

 only way to account for these structures is by supposing them to 

 be eyes once in use at the edge of the mantle, which have been 

 disused as the growth of the shell progressed. There is certainly 

 a dim and worn appearance about them as if they had been for 

 some time out of use. 



At pi. xi., fig. 18, a figure is given of the peculiar way in which 

 the nerve-fibres terminate in the substance of the nacre in this 

 species. 



In addition to this multiplicity of eyes, facetted or otherwise, 

 amongst the Trigonidse, arranged upon the insect type, or numerically 

 similar to the visual organs of insects, we have multiplicity in 

 a way that I have not seen noticed before. I have figured at the 

 end of this paper two tentacular eyes of the common Cerithium 

 eheninum, Brug., to which reference has so often been made in 

 these pages. These eyes are inserted on bulbs or lobes at the 

 outer base of the tentacles. One of them, it will be perceived, is 

 a dark pigmented cup somewhat enlarged at the outer end, on 

 which is inserted a large semi-crystalline eye-ball. At the base 

 of this are two small transparent eyes, and on what is apparently 

 the cornea there is a small crystalline hemispherical protuberance 

 as if a smaller eye were growing on the eye-ball. 



But the lens in this case, though crystalline and transparent is 

 by no means entirely so. It is full of flecks and blemishes or 

 clouds of pigment in the interior, while there is a ring of pigment 

 round the eye upon the cornea. The appearance is represented 

 on pi. v., fig. 6. 



On pi. vi., fig. 7, we have another form of multiplicity. This 

 is an eye of the same species of shell, but very much of the 



