MARINE MOLLUSCA OF THE UNITED STATES. 131 
concavely sloping dorsal line of its auricles, as well as the large 
and peculiar shaped pallets, will readily distinguish it from its 
allies. 
5. T. Tuomsontt, Tryon. Figs. 284-286. 
Proc. Philad. Acad., 280, t. 2, f. 3, 4, 5, 1863. 
Valves higher than wide; anterior auricle moderate, obliquely 
subtriangular; posterior auricle small, not very wide, short, some- 
what reflected outwards, its dorsal margin does not extend so 
high as the beak, nor its basal margin so low as that of the an- 
terior area. Within, the posterior auricle is defined by a strong, 
sharp projection, making a deep sulcus on the exterior surface. 
Pallets obliquely, or sometimes regularly obovate; style short 
and directed backwards; margins of the blade convex. From 
the style an elevated ridge extends around a portion of each side 
of the blade, and is smooth, while the centre portion, extending 
to the end, is lunately striate. Tube not concamerated. 
Three or four feet below low water mark. 
Provincetown and New Bedford, Mass. 
Differs from the other species in the very small proportionate 
size of the posterior auricle, and in its not extending basally as 
low down as the anterior area. 
6. T. cHLoroTIcA, Gould. .Figs. 292-294. 
Invert. Mass., edit. ii. 38, f. 360, 1870. 
Shell minute, subglobose, greenish-white, anterior area very 
large, posterior area quite small, scarcely defined. Pallets with 
lyre-shaped blades, the extreme two-thirds covered by a dark 
encrustation which terminates in two projecting horns. Tube 
lined by a thin gummy or horny coat, and terminating in a con- 
cave calcareous disk with a sort of transverse scar on its outer 
or convex face. ' 
Diameter 3 mill. 
From timbers of ships that have cruised in the Pacific. The 
tubes penetrate the timber across the grain of the wood. On 
account of its several peculiarities of structure, Dr. Gould pro- 
poses for this shell the generic name of Lyrodus. Mr. Jefireys 
believes this species to be identical with 7. pedicellata, Quatre- 
fages. 
