268 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
scales; valves rounded evenly above, only the lateral areas distinct; anterior 
valve with 7 or 8, median with 1, posterior valve with 11 slits; interior of 
valves rose-pink; exterior ashy, marbled with lilac and brown, an obscure 
lilac median line on the medial valves; sculpture of undivided central 
areas formed by two sets of arcuate radiations crossing each other obliquely 
and with the inter-reticulations impressed or punctate, so that an irregu¬ 
larly zigzag effect is produced by the arrangement of the punctations; 
lateral areas irregularly concentrically vermiculate, the spaces between the 
elevated ridges deeply minutely punctate, with somewhat of a zigzag effect 
here also; the sculpture of the anterior valve resembles that of the lateral 
areas; of the posterior valve the mucro is low, subcentral and inconspicu¬ 
ous, the central area sculptured like that of the medial valves, the posterior 
area like the anterior valve; the sutural plates are quite short and the 
sinus smooth and wide. There is no noticeable mucro to the medial valves. 
The peculiar sculpture of this species separates it from those already 
described from this region. In a general way it recalls the very young of 
I. magdalenensis Hinds. (Dali.) 
Type in United States National Museum, No. 109308. Type locality, 
Santa Barbara Channel near Avalon, Catalina, California. 
Range. Santa Barbara to San Diego, California. 
SECTION STENORADSIA Carpenter, 1878 
Ischnochiton acrior (Carpenter) Pilsbry, 1892 
Tryon and Pilsbry, Manual of Conchology, 14:61; PL 14, figs. 86-89. 
Shell much like /. magdalensis , but broader and flatter, with much 
sharper sculpture. Olive-green, pink where worn, or sometimes light flesh- 
colored, with the jugum or central areas often green. Girdle dark ashen 
or olive. The anterior valve is notably concave, as in I. conspicuus. The 
lateral areas are much raised, and sculptured with acute radiating riblets, 
sometimes splitting; central areas having acute longitudinal riblets. Interior 
pink, with a blue spot at the jugal sinus. Sutural plates wide, pink; sinus 
broad, deep, angular. Anterior valves having 13-15, central valves 2-^4, 
posterior valves 10-13 slits. Teeth long, sharp, smooth. Eaves wide, dark- 
blue. Girdle wide, tough, covered with very small, solid, short scales. 
Length, 110; breadth, 55 mm. Length, 75; breadth, 40 mm. (Tryon and 
Pilsbry, Manual of Conchology.) 
Type in Academy of Natural Sciences, Philadelphia, No. 35702. Type 
locality, Point Abreojos, Lower California. 
Range. San Miguel Island, California, to Gulf of California. 
[870 1 
