PRESENCE OP EYES IN SHELLS OE CEETAIN CEITONID^. 17 
ratus j but thè ejes in thè shells of thè Chitonidse may, by 
a little practice^ be readily detected by examining thè dried 
shells directly with a hand lens ; and I bave examined rapidly 
in this way all thè likely looking specimens in thè extensive 
collection in thè British Museum, and that at Montreal, and 
feel pretty certain that no eyes will be found in thè genus 
Chiton, as now distinguished there. In Molpalia, Maugina, 
Lorica, and Ischnochiton, there are apparently no eyes as far 
as a cursory examination has yiclded evidence to me. In 
Chitonellus there are certainly no eyes. 
The arrangement and thè forms of thè eyes vary conside- 
rably in different genera, and these characteristics will pro- 
bably prove of considerable vaine in thè classification of thè 
Chitonidse, which has hitherto proved so difficult a problem. 
The genus Schizochiton is distinguished by having thè 
mantle deeply notched posteriorly, in correspondence with a 
deep median notch in thè hinder border of thè posterior shell 
(PI. IV, fig. 1, c). In Schizochiton incisus thè eyes are 
restricted to single rows traversing thè suturai lines. There 
are six rows of eyes on thè anterior shell, corresponding with 
thè number of marginai notches ; two on each of thè middle 
shells, and six on thè posterior shell — twenty-four rows alto- 
gether, with an average of about fifteen eyes in each, or in all 
360 eyes (see PI. IV, figs. 1, 2, 3, 4, 5). In thè single specimen 
carefully examined all thè rows except one have thè eyes ar- 
ranged in a single straight row at regular intervals, but at thè 
base of one row there are as an exception two eyes side by side. 
There are also in one or two places a very few irregularly scat- 
tered eyes on thè arese laterales, showing that thè condition 
here existing has probably been derived from an ancestral one in 
which thè eyes were not concentrated into lines, but more widely 
diffused on thè shell surface. In one row again, one eye is 
missing from thè spot on which it ought to occur (PI. IV, fig. 
2). The rows of eyes are placed on raised ridges on thè shell 
surface, formed by thè development of tubercles on thè promi- 
nent ridges with which thè surfaces of thè tegmenta are 
ornamented. The eyes in Schizochiton are thè largest I have 
