CONCHIFERA. 



Anisodonta, Deshayes, 1860. 



Type, A. conplanatiim, Dli. Eocene. Paris basin. 



Shell transversely elongated, compressed, inequilateral ; hinge 

 thick ; a large conical and a triangular socket in eacli valve ; 

 ligament external. Anterior adductor scar very small, and 

 comprised between two prominent ribs (one parallel and tbe 

 otber transverse to tbe anterior border) ; posterior Bcar subcir- 

 cular, superficial ; pallial line faint, entire. 



Distribution^ ^ species. Bourbon. 



? Matheria, Billings, 1858. 



Dedicated to Mr. Mather, of the Geological Survey of New 

 York. 



Type, M. tenera, Billings. Trenton limestone, Canada. 



Shell transverse, equivalve ; beaks near the anterior end ; two 

 small obtuse cardinal teeth in the left valve^ and one in the 

 right ; ligament external. 



CoNCHODON, Stoppani, 1865. 



Etymology, conchos^ a shell, and odos, a tooth. 



Type, 0. infraliasicus. Stop. Lower Lias. Lombardy. 



Shell equivalve, symmetrical, very thick, cordiform, closed; 

 beaks large, angulated, involute. Ligament internal, very 

 long, marginal, attached to the posterior half of the hinge-plate. 

 Hinge massive ; in the right valve, one large rounded tooth in 

 front (placed above a dental pit), and two transverse cardinal 

 teeth ; left valve with a large circular socket, bounded below 

 by a curved lamellar tooth ; two transverse and one curved 

 teeth beneath the umbo. 



DiCEROCABDniM:, Stoppani, 1865. 



Etymology, diceras, having two horns, and cardium. 



Shell equivalve, symmetrical, closed, free ; umbones very 

 prominent, elongated, or spiral. Hinge-plate broad, thick, 

 separated by an interval of varying width from the edge of the 

 valve, and prolonged into the umbonal cavity. Left valve with 

 a compressed cardinal tooth, corresponding to a socket in the 

 right valve ; valves furrowed by ligamental grooves. Ligament 

 external. 



Fossil, 4 species. Upper Trias. Lombardy, North-West 

 Himalayas. Yo 



