PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 285 



ROino of the Irregular Chitons, posterior sutural lamincc are found, but 

 these are very exceptional. 



In the vast majority of genera, the side-laminse 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 hau's, 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 aunulated, precisely like the setse of 

 Brachiopods. In certain genera they issue from pores, usually at the 

 wutures, 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 anus is always 

 median and posterior; on each side of it are the sexual openings or 

 fcnestrcc. These may open by several slits or pores directly into the 

 perivisceral cavity, or form the aperture of a sexual duct. The gills, as 

 pointed out by me in 1871, are composed of a row of branchiie, starting 

 froln near the tail, extending a third {])ostic(v), half {))icdiw), or all the 

 way (amhientes) toward the head, each leaflet of which corresponds to a 

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

 conical, with the lamellaj projecting inward, somewhat resembling in 

 outline the shell of Carinaria. The mantle, inside the coriaceous mar- 

 gin of the girdle, often forms a lamina or fringe. A lappet called the 

 ' veil' generally surrounds the front of the rostrum, which has some- 

 times a double veil. The muzzle is semicircular, usually plain, and ex- 

 hibits a tendency to form a lobe at the two posterior corners. The 

 radula is always present. Like the Limpets, Chitons possess a lami- 

 nated crop before the true stomach. The nervous system, beautifully 

 worked out by Brandt t in a jiaper singularly overlooked by most writ- 

 ers, is also comparable with that of Patella vulgata (simultaneously 

 examined and flgiired), though by no means identical. The cephalic 

 ganglia appear to be suppressed, forming another evidence of the degen- 

 eration or want of developement of the cephalic region in this gToup. 

 A valuable paper by Dr. II. von Ihering of Erlangen, I have not yet 

 had access to, but understand that it contains a description of the ner- 

 vous system of Chitons. | 



*Cf. Reiucke, Beitr. zur Bilduiigsges. der Stacbelu, ii. s. w.; Zeitschr. fiir Wiss. 

 Zool. 1858. 



tSt. Petersb. Imp. Acad. Soi. M61anges Biolog. vii, p. 14G, f. 2, 1838, Acanthochiton 

 fiiscicularis. 



t Siuce this paper was written, I have been kindly furnislied by Dr. v. Ihering with 

 copies of his extremely impox'taut work on the ''Anatomy of the Nervons System and 

 the Phylogeny of the Mollnsca," and two valuable papers concerning the Chitonldce 



