PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 285 



some of the Irregular Chitons, posterior sutured lamincc are found, but 

 these are very exceptional. 



In the vast majority of genera, the side-laniinse 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 

 maybe 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 anus is always 

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

 fenestra:. 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 branchiae, starting 

 from near the tail, extending a third (posticce), half (medice), or all the 

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

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

 conical, with the lamellae 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 

 ' veiF generally surrounds the front of the rostrum, w]iich 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 paper singularly overlooked by most writ- 

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

 examined and figured), 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 group. 

 A valuable paper by Dr. H. 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. Reincke, Beitr. zur Bildungsges. dor Stacheln, u. s. w.; Zeitschr. fur Wiss. 

 Zool. 1858. 



tSt. Petersb. Imp. Acad. Sci. Melanges Biolog. vii, p. 14G, f. 2, 1838, Acanthochiton 

 fascicular is. 



X Since this paper was written, I have been kindly furnished by Dr. v. Ihering with 

 copies of his extremely important work on the "Anatomy of the Nervous System and 

 the Phylogeny of tho Mollusca," aud two valuable papers concerning the Chitonidat 



