146 PALAEONTOLOGY OF CALIFORNIA. 



the shell of the latter, it seems to inhabit a long calcareous tube, 

 like the former. At least, there are numerous fragments of tube 

 associated with all the specimens I have seen. The strong in- 

 ternal rib, as large or larger than the rib produced by the um- 

 bonal groove, is a character which will at once distinguish it. 



T. PLENUS, 11. S. 

 PI. 22, Fig. 116. 



Shell thin, broad; apex a third of the distance from the ante- 

 rior end; beaks strongly incurved. Anterior sinus broad, ex- 

 tending almost to the cardinal margin, and gracefully rounded. 

 Umbonal groove deep ; internal rib larger than that produced by 

 the groove. Surface marked by irregular, concentric ribs, which, 

 on crossing the umbonal groove, become linear, and parallel with 

 the curved margin of the anterior sinus. 



Figure, natural size. 



Localiiy : North fork of Cottonwood Creek (Division A.). 



Imperfect casts have been discovered on the Eaneho de San Luis Gonzaga, at 

 the eastern end of Pacheco's Pass, Merced County. These contain shells of about 

 the same shape and size as the above, but in too imperfect a condition for satisfac- 

 tory determination. The tubes are over an inch in diameter; and fragments, eight 

 or nine inches long, showing but little diminution in size, are common. These 

 are always found in fossil wood. 



SOLEN, Linn. 



S. PARALLELUS, n. S. 

 PL 22, Fig. 117. 



Shell elongated, slender, thin ; sides parallel, ends rounded ; 

 anterior end somewhat more abruptly truncated than the poste- 

 rior, and slightly reflected. Surface marked by a few irregular 

 lines of growth. 



