8 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



with such of the described forms as seem nearest alUed is given in 

 the course of our discussion of the next species. 



ISCHNOCHITON (LEPIDOZONA) INTERFOSSA, new species. 



Plate 3, figs. 8-9; plate 5, figs. 5-8; plate 6, figs. 1-4. 



Description. — Animal small, elevated, rather elongate, the sides 

 often nearly straight; maximum diameter about three-fifths the 

 length, or of shell alone less than half the length. 



Anterior valve rounded, conical, its slope very slightly convex. 

 Interior smooth, marked by delicate Imes radiating toward and in 

 correspondence with the marginal slits; teeth slightly roughened at 

 the edges, their outer surfaces faintly striate (pi. 5, figs. 5-6). 



Median valves elevated, beaked, rather sharply carinate, their 

 slopes very slightly convex. Sutural laminae broad; inner margins 

 of same rather abrupt, anterior margins rounded and sloping off 

 more gradually toward the sides; connected across the sinus by a 

 short, delicate, weakly denticulate, concave plate, the minute slits 

 separating the denticles continuing back mto the shell as distinct 

 incised lines, the number of these being uniformly 8. Eaves spongy 

 and quite short (pi. 5, figs. 5-7), 



Posterior valve with conspicuous mucro situated a little in front 

 of the middle; posterior slope nearly straight (pi. 5, fig. 8). SuUiral 

 laminae and intermediate lamina similar to those of the preceding 

 valves, except that there are 9 or 10 of the incised lines. Similar 

 lines radiate from the region of the mucro to the posterior slits. 

 There is a low, semicircular, rounded callus near the margin above 

 the slightly roughened insertion teeth (pi. 5, fig. 6). 



Anterior valve with 11, mtermediate valves with 1-1, posterior 

 valve with 10 slits. 



Entire surface of shell very finely granulose. Anterior valve with 

 immediate apex smooth; elsewhere with from 24-34 low, rounded, 

 radial ribs, separated by shallow but rather sharply cut grooves; 

 the ribs rarely showing a tendency to bifurcation, and typically 

 ornamented with a series of some 6-8 separate, sharply elevated, 

 round pustules, the posterior series on each side containing fewer 

 pustules (4-6) a little larger than the others and projectmg like denti- 

 cles past the margin of the valve. Intermediate valves with lateral 

 areas sculptured like the anterior valve, the 3-4 low ribs intergrooved 

 as above, and bearing 2-3 pustules each, a series of 4-5 larger pustules 

 occurring along the valve margin; central areas ornamented by some 

 20 (valve 7) to 24 (valve 4) straight or slightly curved, faintly nodu- 

 lose, longitudinal riblets on each side, these riblets connected by 

 slightly smaller and less regular transverse crossbars in such a man- 

 ner that in some lights the surface appears reticulate, in others as 

 though cut into lines of small square pits which become more irregu- 



