128 SOLENIDiE. 



bose callous plate; valves equal, divided by two radiating 

 grooves into three portions ; two dorsal valves. 



Penitella (Yalenciennes), Conrad, 1849. 



Distr. — P. penita, Conr. (civ, 65). California. 



Anterior dorsal plates two, placed side by side, posterior to 

 which is a central plate directly over the umbones ; base of the 

 siphons protected by reflected appendages. 



Martesia, Leach, 1847. 



Distr. — 13 sp. World-wide. Fossil; Cretaceous and Tertiary. 

 M. striata, Linn, (civ, 66). 



Yalves lengthened behind when full-grown, by a plain border ; 

 umbonal valves one or two, dorsal and ventral margins often 

 with narrow accessory valves ; surface impressed with one or 

 more furrows. M. striata burrows in hard timber. M. teredin- 

 iformis was found in cakes of floating wax on the coast of Cuba. 

 (G. B. Sby.) 31. Australis in (fossil ?) resin, on the coast of 

 Australia. M. rivicola in timber twelve miles from the sea, in 

 Borneo (fresh-water). M. scutata, 'Eocene, Paris, lines its burrow 

 with shell. 



MARTESIA (restricted). One accessory dorsal plate. 



DiPLOTHYRA, Tryou, 1862. Shell with a double accessory 

 valve ; the principal plate directly over the umbones, with a 

 smaller anterior one adjoining. M. Smithii, Tryon. Staten 

 Island, N. Y., and Chesapeake Bay, burrowing in oyster-shells. 



PHOLAMERiA, Conrad, 1865. The shell has the form of a short 

 Martesia, but without accessory plates ; nothing else, however, 

 occurs in the specific description which would indicate any 

 peculiarity to justify the formation of a new genus. M. triquetra^ 

 Conr. Tert. ; U. S. 



SCHROTERIA, Try On, 1862, Has one preumbonal plate; the 

 anterior hiatus is probably closed. M. cordata, Schroter. 



{Solenacea.) 



Family SOLENID^, 



Shell elongated, gaping at the ends ; ligament external ; 

 hinge-teeth usually 2*3, compressed, the posterior bifid. External 

 shell layer with definite cell-structure, consisting of long prisms, 

 very oblique to the surface, and exhibiting nuclei ; inner layer 

 nearly homogeneous. 



Animal with a very large and powerful foot, more or less 

 c^^lindrical ; siphons short and united (in the typical Solens, 

 with long shells) or longer and partly separate (in the shorter 

 and more compressed genera); gills narrow, prolonged into the 

 branchial siphon. 



