342 



POPULAR CONCHOLOGY. 



it as it opens or closes ; this process is divided within into 

 two diverging teeth; the same valve has a smaller longi- 

 tudinal septum in its middle, between the muscular im- 

 pressions. The beak of the (upper) convex valve is in 

 some species produced considerably. — Fossil. 



" All the species have striated surfaces, and many have 

 long spines, often tubular on the outside, and numerous 

 little spine-like projections within."* 



Dayidsonia. Bouchard. — Shell inequivalve, adhering 

 by its lower valve, which is very thick ; area not separated 

 from the rest of the valve; deltidium filled up by the 

 solid matter of the shell ; hinge with two strong teeth 

 like those of Terebratula, with two deep anterior muscular 

 impressions placed between the dental plate ; two pos- 

 terior muscular impressions in the form of elevated massive 

 cones. — Fossil. 



This description applies only to the lower valve, M. 

 Bouchard not having seen the upper. 



Orthis. Dolman. (Orthotetes Fischer; Gonambo- 

 nites, Orthambonites, Bonites, Hemipero- 

 nites, Klitambonites Pander.) — Shell with 

 hinge rectilinear, with umbones distant ; the 

 larger valve with a transverse basal smooth 

 area, with a triangular pit. f — Fossil. 0rthis hasalis - 



Orthisin a. & Orb. — Shell free, not perforated, oval 

 or angular, transverse, depressed, inequivalve ; the pierced 

 valve very large, sub-conical, with a very large area ; the 

 unpierced valve nearly flat, with a smaller area than the 

 other ; opening round, placed below the beak of the upper 



* Sow. Mineral Conch. 



f Sow. Conch. Manual. 



