ISCHNOCHITON. 127 



I. COOPERI Carpenter, n. sp. PL 26, figs. 27-30. 



Shell oval and elevated, with angular dorsal ridge and straight 

 side-slopes. Sculpture like /. mertensii. Color olivaceous, or dull 

 earthy brown, indistinctly clouded more or less with light blue, 

 especially upon the side areas. The lateral areas are raised, and 

 bear irregular rows of rounded pustules, the young having four rows, 

 the adult 6 to 8. A strong lens reveals a fine, subobsolete granula- 

 tion of the nearly flat surface between the pustules. The central 

 areas have a fine but distinct and even radial striation, over which 

 run acute narrow raised threads parallel to the dorsal ridge ; upon 

 the ridge these threads are seen to be more or less diverging, 

 especially upon the second valve. The end valves are radially 

 ridged, the ridges bearing elongated pustules, or showing scars 

 where such pustules have been. Mucro low, flat. 



Interior bluish, the valves marked under their umbones with dark 

 olive. Head valve having 11, central valves 1, tail valve 11 slits ; 

 teeth roughened but rather sharp; eaves wide, dark, minutely 

 punctulate, but solid, not spongy. 



Girdle compactly covered with small imbricating, deeply striated 

 scales)(fig. 28). Length 40, breadth 24 mill. 



Santa Cruz and Bolinas, California. 



This species closely resembles I. mertensii in shape and sculpture, 

 but it differs entirely in the smaller, distinctly striated or grooved 

 scales, in the sombre coloring both outside and within, in the closer 

 pectination of the inter-liral spaces of the central areas, and in the 

 more developed microscopic granulation of the lateral areas. It 

 cannot be claimed that all or any one of these characters is invari- 

 able, but the balance of them taken together, seem to indicate that 

 differentiation has proceeded to a stage we may call specific. The 

 following species is allied to this in coloring, but differs sufficiently 

 in sculpture, as well as in the girdle-scales. 



The details of sculpture and the scales of all three species are 

 drawn to the same scale. The third valve of /. cooperi is shown in 

 figs. 29, 30. It must be remembered that the valves behind this, and 

 usually even the third, have less markedly divergent sculpture than 

 the typical form here shown. 



The specimens herein described and figured are from Bolinas, 

 north of San Francisco, and since no diagnosis has been published 

 hitherto, these will be considered the typical specimens. Carpenter's 

 specimens were from Sta. Cruz. 



