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. Thomsonii, Tryon. Figs. 284-286. 



Proc. Philad. Acad., 280, t. 2, f. 3, 4, 5, 1863. 



Yalves 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. Prom 

 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. 33, 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 ivood. On 

 account of its several peculiarities of structure, Dr. Gould pro- 

 poses for this shell the generic name of Lyrodus. Mr. Jeffreys 

 believes this species to be identical with T. pedicellata, Quatre- 

 faares. 



