39 



Cretaceous, though I am inclined to think, from their gene- 

 ral characters taken as a group, that it belongs to that epoch. 

 The presence of the subgums Goniomya goes against the 

 conclusion that it may be Tertiary, as that type of Myacida 

 is unknown in newer rocks than those of the Cretaceous 

 system, and is most common in the Jurassic. The genus 

 Trigonia is, I believe, also unknown in the Tertiaries, though 

 existing in our present seas. 



Full illustrations and descriptions of these fossils will 

 soon be published in Lieut. Williamson's report. 



DESCRIPTIONS OF SPECIES. 



Nucula Traskana. — Triangular-ovate, both extremities 

 rather narrowly rounded ; cardinal border sloping from the 

 beaks at an angle of about one hundred degrees, base form- 

 ing a broad gentle curve ; beaks nearly central, rather ele- 

 vated ; surface (of worn specimens and casts) showing re- 

 mains of concentric lines ; muscular impressions shallow ; 

 hinge having in each valve about fifteen teeth on each side 

 of the small ligament ary pit. 



Length *27 inch ; height *18 inch; breadth '16 inch. 



The only specimens of this species I have seen are casts, 

 or so much worn as to show imperfectly the surface charac- 

 ters. As near as can be ascertained from these it appears 

 to have had rather distinct concentric lines of growth, and 

 judging from the form of the anterior slope of the cardinal 

 border, there was probably a well-defined ovate lunule. It 

 will perhaps be identified by its ventricose triangular ovate 

 form, and nearly central beaks. 



The specific name is given in honor of Prof. John B. Trask, 

 State Geologist of California. 



Arca (Cucidlaea) equilateralis. — Oblong-oval 3 gibbous 

 in the middle, and sub-angular down the posterior umbonial 



