CHITONIDA, 307 
pass forward from each of the posterior seven valves under the 
valve in front. The sutural laminz are also destitute of the 
porous layer. The sinus is either open, or part of the jugular 
area projects forward between the sutural laminz, forming a false 
apex; or a keystone-like piece, either solid, or fimbriated like 
the teeth of a comb, may exist between the laminz and _ partly 
fill the sinus. The sinus posticus is the wave, notch or inden- 
tation which in some genera is found in the posterior edge of 
the posterior valve. In some of the irregular Chitons posterior 
sutural laminz are found, but these are very exceptional. 
In the vast majority of genera, the side lamine of insertion 
have only one slit on each side of the valve; occasionally a valve 
may be abnormal in a regular species, and the number of slits in 
the anterior and posterior valves may vary within moderate 
limits. 
The girdle (zona), which is distinct from the true mantle, is 
variously ornamented with scales, bristles, spines, down or hairs, 
either singly or combined, which exhibit most beautiful forms 
tolerably constant in generic groups, and worthy of a special 
and exhaustive research. These may be solid or hollow, shelly 
or keratose, single or combined in bunches, and in some forms 
are hollow and annulated, precisely like the setz of brachiopods. 
In certain genera they issue from pores, usually at the sutures, 
and these pores have a certain value as a systematic character, 
but much less than has been assigned to them by some authors. 
The Chitons in the adult condition are destitute of eyes or 
tentacles, and exhibit evidences of degradation anteriorly. The 
intestine is straight and the anus is always median and posterior; 
on each side of it are the sexual openings or fenestre. These 
may open by several slits or pores directly into the perivisceral 
cavity, or form the aperture of a sexual duct. The gills are 
composed of a row of branchiz, starting from near the tail, 
extending a third (postic), half (media), or all the way (ambi- 
entes) towards the head, each leaflet of which corresponds to a 
whole branchial plume, such as is found in Acmea. Each single 
gill is conical, with the lamelle projecting inwards, somewhat 
resembling in outline the shell of Carinaria. The mantle, inside 
the coriaceous. margin of the girdle, often forms a lamina of 
fringe. A lappet.called the ‘“‘veil”’ generally surrounds-the front 
of the rostrum, which has.sometimes a double veil. The muzzle 
is semicircular, usually plain, and exhibits a tendency to. form a 
lobe at the two posterior corners,. The radula.is always. present 
and, as in the limpets, is very long; lingual teeth three, median 
small, laterals large, with dentated hooks, uncini five trapezoidal, 
one of them erect and hooked. Like the lim pets Chitons possess 
a laminated crop before the true stomach. The nervous system 
beautifully worked..out.. by Brandt. im a. paper. “singularly oyer- 
