2S4 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM 



lu many genera tliere is a small portion of peculiar sculpture marked 

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

 jiKjali., and corresponds to the sinus or space between tlie inner termina- 

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

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

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



Fig. a. — Chiton olivaceus Sowerby. A, autcrior, B, middle, aud C, j>osterior valve, 

 seen from outside; D, E, F, the same valves, from witliin; g, sinus jugali, between 

 the two anterior siitdral laminae ; h, h, denies, or teetb between tbe notches or slits in 

 the laminiB of insertion; (, k, slits in lamina of insertion; ?, outer end of line separa- 

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

 separating the central area of the tail-plate from the posterior portion : the augnlation 

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



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

 ina} of insertion and notches; B, the middle valve, from above ; g, sinus jugali, in front 

 of thejugum, or area jugali, and between the two sutural lamina); i, I, notches between 

 the sutural and iusertional laminie, and forming 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, eiul of line separating the areas ; D, anterior valve, from within, 

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

 within; ^, jugal sinus, between the sutural lamina; ; i, notch, between sutural lamina 

 and side-lamina of insertion. 



or part of the jugular area projects forward between the sutiu-al laminae, 

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

 like the teeth of a comb, may exist between the laminte aud partly fill 

 the siuus. The sinux posticus is the wave, notcli, or indeutation which 

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



