368 Dr. JrE. Gray on some Families of SivaJve Shells. 



There is no appearance of the angular lines found in the former 

 section, and the cartilage or ligament, when it dries, splits into 

 longitudinal filaments extended from one umbo to the other. 



I. The hinge-line linear, straight ; the teeth divided into numerous, 

 small, nearhj equal-sized, transvei^se ci'ests ; the umhones 

 separated by a broad lozenge-shaped area, with a series of 

 distant grooves regularly diverging from the hinge-margin 

 towards the umbo ; jnargin of the shell entire, often gaping 

 below. Periostraca paleaceous. 



1. Litharca, Gray, Syn. B. M. 1840, 155 ; 1844, 81. Shell 

 wedge-shaped, closed beneath, obliquely truncated behind, elon- 

 gated and rounded in front; umbo posterior; hinder hinge- 

 tooth absent; anterior hinge-tooth very long, linear, divided 

 into many equal plates ; margin smooth. Periostraca paleaceous. 



L. Lithodomus = Byssoarca L., Sow. 



Not having the animal, there is some difficulty in respect to 

 the natural position of this shell. I have here regarded the 

 short truncated end as the posterior, because I think I can ob- 

 serve the large oblong scar of the hinder pedal muscle under the 

 inner margin of the truncated portion. 



2. Area, Gray, /. c. 155 = Byssoarca, Swainson. Shell ob- 

 loug, subquadrate, gaping beneath ; hinge-margin rather pro- 

 duced at each end ; umbo subanterior, curved ; front cardinal 

 tooth moderate, divided into small equal crests like the hinder 

 one; the scar of the hinder pedal muscle very large, oblong, 

 triangular, on the hinder half of the dorsal surface of the cavity 

 of the shell. Periostraca paleaceous. 



a. Teeth small. Area Nose, A. pacifica, A. truncata, A. navicu- 

 lars, A. angulata — if these presumed species are more than 

 local varieties. 



b. Teeth larger. A. zebra. 



II. The hinge-line broad, more or less curved ; the teeth ivider at 

 the distal ends ; the crest of the inner portion small, trans- 

 verse, of the distal portion larger arid more or less oblique or 

 longitudinal ; the umbones more or less separated by an elon- 

 gate area, marked with angular concentric cartilage-grooves, 

 or small, ivith slight cross-lines. 



A. Shell radiately striated or nearly smooth ; lower margin entire 

 or subdentate, sometimes gaping; cardinal teeth subequal ; 

 the umbonal area marked ivitJt angidar concentric cartilage- 

 grooves. 



3. Trisis, Oken; Gray, Syn. B. M. 1840, 155. Shell sub- 



