128 SOLENIDA. 
bose callous plate; valves equal, divided by two radiating 
grooves into three portions ; two dorsal valves. 
PENITELLA (Valenciennes), 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. 
MarrestA, Leach, 1847. 
Distr.—13 sp. World-wide. Fossil; Cretaceous and Tertiary. 
M. striata, Linn. (civ, 66). 
Valves 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. WM. striata burrows in hard timber. J. teredin- 
iformis was found in cakes of floating wax on the coast of Cuba. 
(G. B. Sby.) MM. Australis in (fossil?) resin, on the coast of 
Australia. MM. rivicola in timber twelve miles from the sea, in 
Borneo (fresh-water). J. scutata, Eocene, Paris, lines its burrow 
with shell. 
MARTESIA (restricted). One accessory dorsal plate. 
DIPLOTHYRA, Tryon, 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. WM. triquetra, 
Conr:* ‘Tert..-C! 8. 
SCHROTERIA, Tryon, 1862. Has one preumbonal plate; the 
anterior hiatus is probably closed. J. cordata, Schroter. 
(Solenacea.) 
Famity SOLENID.. 
Sheil 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 
cylindrical; 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. 
