144 Prof. H. N. Moselcy on the 



by projecting beyond the margin of the cornea all round forms 

 an iris of less diameter than the latter. 



A perfectly transparent, hyaline, strongly biconvex lens is 

 fitted in behind the iris- aperture. The lens is composed of 

 soft tissue, and dissolves in strong acetic acid, gradually but 

 completely, showing a fibrous structure in the process. 



The optic nerve at some distance from the eye is a com- 

 pact strand ; but within the very long tube continuous with the 

 choroid — the nan-ow part of the pear — its numerous fine fibres 

 are much separated from one another and loose. The retina 

 is formed on the type of that of Helix^ and not, as might 

 have been expected, that of the dorsal eyes of Oncidium. It 

 is not perforated by the optic nerve, but is composed of a 

 single layer of very short but extremely distinct and well- 

 defined rods, with their extremities directed towards the light. 

 Beneath them is a layer or several layers of nuclei amongst 

 the ultimate ramifications of the nerve. 



Not all the fibres of the nerve entering the eye-cavity 

 proceed to the retina. A large number of the peripherally- 

 placed fibres pass outside the retina all round, and, perforating 

 the choroid at its outer margin, end at the surface of the shell, 

 all round the area occupied by the cornea. They terminate in 

 small plugs of tissue, corresponding to those minor organs 

 of touch universally distributed over the shell in the smaller 

 pores already described — being, in fact, exactly similar and 

 identical structures with these. They apparently form a sen- 

 sitive zone round each eye, and they arise from the optic nerve 

 just as do the other minor sense-organs from the nerves of 

 the larger organs of touch. The choroid sacs of the eye 

 show a curious open fold or gutter leading from the bulb 

 superficially along the stalk of the pear, recalling curiously 

 the choroid fissure. 



Tn some genera of the Chitonidge eyes are entirely absent. 

 This is the case with the genus Chiton. The shell in Chiton 

 is perforated ; the usual small and large pores and the small 

 and large touch-organs are present, but I have as yet found 

 no trace of eyes. I have examined especially C. magnijicus 

 and C. marmoratus. In Moljmlia, Maugina^ Lorica, and 

 Isclinochiton there appear to be also no eyes so far as a cur- 

 sory examination has yielded evidence to me. 



The arrangement and forms of the eyes vary much in diffe- 

 rent genera, and will probably prove of great value in classi- 

 fication, which has hitherto proved so difficult a problem. 



The genus ScMzocMton is distinguished by having the 

 mantle deeply notched posteriorly in correspondence with 

 a deep median notch in the posterior shell. In 8chizochiton 

 mctsxis the eyes are restricted to single rows traversing the 



