ISCHNOCHITON. 97 



sharp, curiously marked by a long, narrow white line which may 

 be accidental. Central areas having very strongly marked rugose - 

 granulations ; falling irregularly into corrugations, but made up of 

 grains. Lateral areas moderately raised, with the same grain- 

 wrinkling passing over about four very blunt obsolete ribs ; end 

 valves obsoletely radiated. Interior: posterior valve having 11, 

 central valves 1, anterior valve 10 slits; teeth sharp, short, thin ; 

 sinus broad but shallow, very straight, smooth, not channelled ; 

 eaves moderate, rather close to the teeth. Length 111, breadth 4i 

 mill. ; divergence 103. 



I. CURTISIANUS E. A. Smith. PL 24, fig. 6 (enlarged). 



Shell oval, flattish, of a dirty dark grayish color, having a con- 

 spicuous black broadish line from end to end down the middle of 

 the back, with a pale one on each side of it, and with the mantle 

 patched alternately light and dark, irregularly granulated through- 

 out. Valves arched, not carinate at the vertex, with very indistinct 

 lateral areas, exhibiting strong concentric lines of growth, especially 

 conspicuous at the sides and anterior margins. Front plate well 

 curved anteriorly, the posterior margin being broadly sinuated. 

 Second plate longer than the other intermediate valves, slightly 

 incurved on each side, the central outcurved point in front, rather 

 peaked in the middle of the hinder edge. The third, fourth, fifth, 

 sixth and seventh valves are very short in comparison with their 

 width in proportion as 1 is to 3. The jugal sinus is large and arcu- 

 ate. Last valve of a narrow acutely elliptic form, with the mucro 

 probably near the center. Interior of the plates greenish-blue, 

 stained dark brown in the middle. Laminse of insertion in the 

 front plate with about ten notches at unequal distances, leaving 

 different sized teeth between them, which are striated on both sides, 

 but more strongly externally, their edges being sharp, but not 

 smooth. The central plates have a single minute notch on each 

 side, the insertion lamina being comparatively smooth on the upper 

 surface and marked with a small brown spot on each side against 

 the edge of the valve. Tail plate much thickened within along the 

 posterior edge, which is roughened by fine cross stria?, there being 

 no prominent teeth, and of course no notches. Mantle margin cov- 

 ered with small subi rubricating oval granules. Length without 

 margin 16 mill. ; width of fourth plate 9. 



