2 THE NAUTILUS. 



II. Subgenus Petrasma Dall, nov. Ligament not exposed inter- 

 nally in front of the chondrophore ; type S. borealis Totten. 



A. Chondrophore supported by two strong props with a deep 

 cavity between them. 



1. S. borealis Totten. 



2. S. velum Say. 



B. Chondrophore with an anterior prop extended as a slender rib 

 in front of the adductor scar ; no posterior prop. 



1. S. occidentalis Deshayes. 



2. S. pusilla Gould. 



3. S. panamensis Dall, n. sp. 



C. Chondrophore without props. S. volvulus Carpenter. 



III. Subgenus Acharax Dall, nov. 



Ligament opisthodetic, wholly external, visible internally only 

 where it crosses the gap between the margins of the valves. Nymphg 

 without props. Type S.johnsoni Dall. 



1. S.johnsoni Dall. 



2. S. patagonica E. A. Smith. 



3. S. agassizii Dall, n, sp. 



4. S. ventricosa Conrad, fossil. 



5. S. grandis Verrill and Bush. 



NOTES. S. togata (Poli) auct., and S. mediterranealuamarck, are 

 synonyms of S. solen. S.jctponica Dunker, is the adult of S. pusilla 

 Gould. S. macrodactyla Rochebrune and Mabille is probably identi- 

 cal with S. patagonica, though the unique type of the latter seems 

 pathologically callous dorsally. 



S. protexta Conrad, if not the young of S. ventricosa Conrad, from 

 the Miocene of Oregon, probably belongs to Petrasma. 



S. occidentalis Deshayes, is common to the Mediterranean, West 

 Indies and Gulf of Mexico. S. panamensis extends from off Santa 

 Barbara, Cal., to Panama Bay ; S. valvulus Carpenter, from San 

 Pedro, Cal., to the Gulf of California ; S. agassizii from off Tilla- 

 mook Bay, Oregon, south to Aguja Point, Peru, in 1036-1800 

 fathoms, and S. johnsoni Dall, from Puget Sound to Panama Bay 

 in 60 to 1740 fathoms. 



The rarity of these species, and the fact that they usually break up 

 into fragments in drying, are probably the reasons why the remark- 

 able differences between the hinges of the different species have not 

 previously attracted attention. 



