CALLISTOCHITON. 269 



Adams, of which there were no duplicates to compare. It offers a 

 still more marked transition to Callochiton, the margin of the pos- 

 terior valve being somewhat pectinated by the great projection of 



the ribs. (Cpr.~) 



Panama (C. B. Ad.) 



Ischnochiton (? var.) expressus CPU., P. Z. S. 1865, p. 275. 



This species, like the last, is not known to me by specimens, the 

 above description. and notes being from Carpenter's paper in the P. 

 Z. S. In his MSS. Carpenter places it in the smooth scaled section 

 of Ischnochiton, and gives some additional notes, as follows : " This 

 shell is almost exactly like the young of I. serratus, agreeing not only 

 in the peculiar and beautiful sculpture, but even in the mantle scales. 

 Yet it differs remarkably in the abnormal, narrow pointed sinus, in 

 which respect it resembles /. tectiformis. One of these species is 

 probably the C. clathratus of C. B. Ad. (Pan. shells, p. 242), and 

 possibly of Reeve." 



C. DECORATUS Carpenter, n. sp. PI. 58, figs. 17-20. 



Shell oblong, moderately convex, obtusely subangular along the 

 dorsal ridge, the side-slopes slightly convex. Surface rather shining, 

 varying in color from olive-buff to dark green, uniform or having 

 darker flammules and spots. 



Apices of the median valves eroded. Lateral areas raised, each 

 rendered bicostate by a central sulcus, and having a few uneven 

 longitudinal impressed grooves, giving it a terraced aspect. Central 

 areas having numerous parallel longitudinal ribs, the intervals 

 closely latticed across, this sculpture obsolete on the ridge, where there 

 is a broad V-shaped smooth area, which rarely shows a few diverging 

 subobsolete ribs. Anterior valve having 11 rounded ribs. Posterior 

 valve less elevated than the anterior, highest at the front margin, the 

 mucro depressed, post-median ; posterior area having 9-10 rounded 

 ribs. 



Interior bluish, generally marked with olive or green at jugum, 

 bases of the sutural-plates, and slit-rays. Sutural plates wide, 

 separated by a square sinus notched at each side. Anterior valve 

 having 9 or 11, central 1, posterior valve 9-12 slits ; teeth rather 

 sharp, smooth ; slit-rays distinct and porous. Eaves narrow. 



Girdle narrow, ashy-brown, covered with minute, striated, closely 

 imbricating scales. 



Length 20, breadth 9-10 mill.; divergence about 110. 

 Todos Santos Bay and near San Tomas River, Lower California. 



