284 TEOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



In many genera there is a small portion of peculiar scnlptiire marked 

 off along the ridge of the median line of the back. This is the area 

 jugali, and corresponds to the simis or space between the inner termina- 

 tions of the two anterior sutural lamimc which pass forward from each 

 of the posterior seven valves under the valve in front. The sutural 

 laminae are also destitute of the porous layer. The sinus is either open, 



A. B. 



Fig. a. — Chiton olivaceus Sowerby. A, anterior, B, middle, and C, posterior valve, 

 seen from outside; D, E, F, the same valves, from within; g, sinus jiigali, between 

 the two anterior sutural lamin* ; 1i, h, denies, or teeth between the notches or slits in 

 the laminaj of ins:^rtion; j, &, slits in lamina of insertion; I, outer end of line separa- 

 ting the area lateralis of the middle valve from the area centralis ; m, end of the line 

 separating the central area of the tail-plate from the posterior portion ; the angulation 

 of this line in the median line of the animal forms the mucro. 



Fig. B. — Enoplochifon niger Barnes. — A, anterior valve from above, showing the lam- 

 inae of insertion and notches; B, the middle valve, from above ; g, sinmjiigaH, in front 

 of the jugum, or area jugali, and between the two sutui-al lamimc; i, I, notches between 

 the sutural and iusertional laminie, and formiug the ends of the line forming the an- 

 terior border of the lateral and posterior border of the central areas; C, tail-plate; 

 g, sinus jugali ; m, end of line separating the areas; D, anterior valve, from within, 

 showing teeth (7i, h) and notches of lamina of insertion; E, middle valve, from 

 within ; g, jugal sinus, between the sutm-al laminai ; t, notch, between sutural lamiuu 

 and side-lamina of insertion. 



or part of the jugular area projects forward between the sutural lamintie, 

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

 like the teeth of a comb, may exist between the lamin?e and partly fill 

 the sinus. The sinus posticus is the wave, notcli, or indentation which 

 in some genera is found in the posterior edge of the posterior valve. In 



