CHITONID^. 33T 



pass forward from each of the posterior seven valves under the 

 valve in front. The sutiiral laminae are also destitute of the 

 porous la3'er. The sinus is either open, or part of the jugular 

 area projects forward between the sutural laminae, forming a false 

 apex ; or a keystone-like piece, either solid, or fimbriated like 

 the teeth of a comb, may exist between the laminas 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 laminae are found, but these are very exceptional. 



In the vast majoritj^ of genera, the side-laminse of insertion 

 have onl}' 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), w^hich is distinct from the true mantle, is 

 variouslj^ 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 setae 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 fenestrse. 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 branchiae," starting from near the tail, 

 extending a third (posticae), half (mediae), or all the way (ambi- 

 entes) towards the head, each leaflet of which corresponds to a 

 whole branchial plume, such as is found in Acmaea. Each single 

 gill is conical, with the lamellse projecting inwards, somewhat 

 resembling in ovitline 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 tw^o posterior corners. The radula is alw-ays 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 limpets Chitons possess 

 a laminated crop before the true stomach. The nervous system 

 beautifully worked out by Brandt in a paper singularly over- 



