MARINE MOLLUSCA OF THE UNITED STATES. 97 



3. D. PLiocENUM, Toumey and Holmes. Fig. 197. 

 Pliocene Foss. So. Car., 105, t. 25, f. 2. 1857. 

 Shell slightly ciirved, marked ])y about thirty-eight very indis- 

 tinct ribs, which become obsolete towards the base ; lines of 

 growth numerous, indistinct ; aperture orbiculai-. 



(Living?) South Carolina. 



Family CHITO^^ID^. 



Head surrounded by a semicircular veil or hood; e3'es and 

 tentacles none; mouth with cartilaginous jaws; gills in a series 

 of lamelliB, between the mantle and foot round the sides and pos- 

 terior, part of the body; foot oblong, rounded at each end. Lin- 

 gual ribbon long and linear, with numerous trausvex'se series of 

 teeth. 



The Chitons are abnormal mollusks in many respects. Their 

 shell in eight separate but connected pieces gives tliem an articu- 

 lated appearance ; the heart is central ; the reproductive organs 

 symmetrical, with two orifices, and the sexes united ; the intes- 

 tine is straight, and tlie anal orifice posterior and median. 



Notwithstanding these resemblances to the annelids they are 

 believed to be more closely related to the mollusca. 



Genus CHITON, Linnaeus. 



Sjst. Nat., edit. x. 1708. 



1. C. MENDiCARius, Mighels and Adams. Fig. 19S. 



Best. Journ. Nat. Hist., iv. 42, t. 4, f. 8. 1842. 

 G. Hanleyi, Bean. Thorpe's Brit. Mar. Conch., 263. 1844. 



Shell cinereous, with dark clouds, long, oval with obtuse dorsal 

 ridges ; surface with elevated dots or granules, disposed in longi- 

 tudinal lines^ except towards the margin, where tliey are irregular 

 and larger ; no visible concentric striae ; triangular areas very in- 

 distinct, outer whorls small, margin coriaceous, red. 



Length 25, breadth 10 mill. 



Mai}ie, northwards. {Eur.) 



2. C. APicuLATUS, Say. Fig. 199. 



American Conch., No. 7. 1834. 



Shell oblong-oval, convex ; valves obtusely carinate, the central 



portion of the posterior margins becoming slightly beaked with 



age. Lateral areas triangular, studded with numerous rounded 



tubercles, obsolete towards the apices, more numerous towards 



7 



