PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 285 



some uf the Irregular Chitons, posicrior sninntt lainhuv are found, but 

 these are ver^' exceptional. 



In the vast majority of genera, the side-Iamiiue of insertion have only 

 one slit on each side of the valve; occasionally a valve may be abnormal 

 in a regular si^ecies, and the inimber of slits in tlie anterior and posterior 

 valves may vary within juoderate limits. 



The girdle {zona), which is distinct from the true mantle, is \ariously 

 ornamented with scales, bristles, spines, down, or hairs, either singly or 

 combin(Hl, 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, x)recisely like the setce 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 it are the sexual openings or 

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

 peri\isceral 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 {jjosticcv), half {mecUw), or all the 

 way {ainbientes) toward the head, each leaflet of which corresponds to a 

 whole branchial plume, such as is found in Acmim. 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 fiinge. A lappet called the 

 ' veil' generally suiTounds 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 nervons 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 cAidence 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 

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

 vous system of Chitons. | 



*Cf. Reiucke, Beitr. ziir Bildungsges. der Staclieln, u. s. w.; Zeitsclir. fiir Wiss. 

 Zool. 1858. 



t St. Petcrsb. Imp. Acad. Sci. Mdlangea Biolog. vii, p. 14G, f. 2, 18G8, Amnthochiton 

 fascicular is. 



X Since this paper was written, I Liavo been kiu<lly lumislied by Dr. v. Ihering with 

 copies of his extremely important work on the *'^\jiatomy of the Nervous System and 

 the Phylogeny of the MolUisca," and two valuable papers concerning the Chitonidx 



