86 



dorsally sub compressed, latero-ventrally somewhat 

 obtusely rounded. Section of the shell obovate. 

 The above figures of natural size. Locality, Chico 

 Creek, Cal. 



Description of three new species of the Genus Pla- 



giostoma, from the Cretaceous rocks of Los An- 



geles. By Dr. John B. Trask. 



Up to the present time, no mention has been made 

 of the occurrence of the Cretaceous rocks in this 

 State. The researches of F. Roemer, in Western 

 Texas and New Mexico, demonstrated their existence 

 to the middle and southern portions of this territory, 

 at which point he left them. 



A late traveller, Julius Froebel, extended his ob- 

 servations over the ground of Roemer in part, and 

 continued the same into California. In a conversa- 

 tion with the former gentleman, in 1854, upon the 

 fossils of New Mexico and westward of that country, 

 he intimated the probable existence of the Cretaceous 

 rocks west of the Colorado. The fossils collected by 

 him west of that point, were not sufficiently well de- 

 fined, however, to base a positive conclusion upoD, 

 and place the matter beyond a doubt. I feel satisfied 

 at the present time that most of those fossils are re- 

 ferable to that period, and that the opinion he then 

 advanced was well grounded. 



Since that time I have been fortunate enough to 

 discover fossils, of as much antiquity at least as those 

 of Western Texas, and probably still lower in the se- 

 ries, the rocks containing them forming the coast ol 

 the Pacific Ocean in this State. There can be no 

 doubt therefore at present, that the Cretaceous rocks 

 extend from the Atlantic to the Pacific. 



The rocks in which these fossils are found, occur 

 at San Pedro, in the county of Los Angeles, imme- 

 diately upon the coast, and underlying the superficial 

 tertiary beds (lately denominated recent formations 

 by Mr. T. Conrad) of this locality. I have some hes- 

 itancy in placing these rocks so high up in the geolo- 

 gical series, as the position here assigned them, but 

 as the associated fossils are as yet somewhat obscure 

 ;ind ill denned, it is thought best to place them here 

 for the present, or until farther examination of their 

 fossils shall classify them otherwise. 



The tertiary deposits at this place are about thirty 

 feet in depth, and have a low northerly dip ; about 

 twenty feet of this deposit is made up of beds of fos- 

 sil diatomacea, the upper stratum of which is white, 

 and similar in appearance to that found at Monterey, 

 but much less dense, the forms differ but little from 

 hat deposit. 



Beneath the tertiary beds, a dark, soft, marly de- 

 posit crops out but a tew feet above tide water, hav- 

 ing a northerly dip of about fifty degrees, and ex- 

 tending along the beach for three-fourths of a mile. — 

 They contain the fossils described and figured below, 

 associated with small crustaceans and coralines, the 

 latter too fragile to admit of demonstration. Con- 

 formable to these beds, along the shore to the west 

 and north, are beds of a yellowish and buff-colored 

 limestone, resembling a coarse variety of lithograph- 

 ic stone, containing fossil crustaceans of small size. 

 none of which have yet been found sufficiently per- 

 ;cct to admit of a description. 



The fossils here described and figured, are all from 

 very perfect casts ; the fine striae and small fold 

 upon the auricles, are as perfect as they possibly 

 sould have been upon the shell originaljy. 



The fossils I have refered to the genus Plagiostoma 

 of Sowc-rhy, but having the figures of but two spe- 

 cies of that genus, and no description whatever, I am 

 compelle I to omit reference to those heretofore des- 



cribed, that may simulate these in form or other- 

 wise. 



Plagiostoma (Sow) Pedrona. Trask. 



Plate III. Fig. 1. 

 Shell compressed ; sub-triangular, with eight or 

 nine flatly rounded concentric annulations, which 

 are nearly as distinctly marked on the interior of the 

 vajve for about half the height from the ventral 

 margin ; (see fig. 1. A.) beaks acute at the apex, and 

 as high as the line of the auricles; anterior auricle 

 rounded in front, and has a small, thin fold extend- 

 ing from the umbone to the centre of its anterior 

 margin, and is covered with about thirteen small, 

 rounded, radiating striae, which converge at the 

 beak ; beaks at the anterior third ; sub-acute ; ante- 

 rior margin rounded, and somewhat produced ; ven- 

 tral margin smoothly arched ; posterior margin 

 rounded, becoming slightly arcuate toward the dor- 

 sam of the shell ; posterior auricle angulate, and ob- 

 tusely truncate posteriorly. Length one and eight- 

 twentieths of an inch ; height one and four-twentieths 

 of an Inch. Locality, San Pedro, Cal. 

 P. annulatus. Trask. 



Plate III. Fig. 2. 



Shell compressed ; obliquely rounded ; five to sev- 

 en broad concentric annulations ; anterior margin 

 obtusely rounded ; ventral margin rounded ; poste- 

 rior margin somewhat produced ; ventrally, and be- 

 coming slightly arched toward the dorsum; beak at 

 the anterior half, and slightly raised above the line 

 of the auricles ; anterior auricle angular, with about 

 twelve fine striae divergent from the umbones ; pos- 

 terior auricle subangulate, and truncate posteriorly. 

 Length, one and five-twentieths of an inch ; height, 

 one and three-twentieths of an inch. Locality, San 

 Pedro, Cal. Found with the preceding. There were 

 three specimens of this species found — the left valve 

 was used for the figure from its being the most per- 

 fect. 



P. truncata. Trask. 



Plate UI. Fig 3. 



Shell compressed ; rotund-quadrate; about eight 

 flattened, slightly rounded, concentric annulations; 

 anterior margin rounded, and somewhat obtuse be- 

 low the middle ; ventral margin orbicular ; posteri- 

 or margin bluntly rounded ; posterior dorsal margin 

 subangulate ; beaks acute above the line of the auri- 

 cles ; anterior auricle rounded in front, striate, and 

 has a small fold extending from the umbones to its 

 anterior margin above its middle, posterior auricle 

 angulate and obtuse posteriorly ; beaks anterior to 

 the middle. Length, one and one-tenth inches ; — 

 height, one inch. Locality, San Pedro, Cal. Found 

 with the preceding species. 



San Francisco, March 24, 1856. 



President in the chair. 



Donations to the Cabinet. 



From Dr. J. A. Veatch, plants from the vicinity 

 of Lick Springs, Shasta county. 



From Mr. J. P. Haven, a large collection of marine 

 shells, from the Islands of the South Pacific ; the 

 skull of a Barbaroussa ; specimens of Corals; a Fly- 

 ing Fish, and articles manufactured by the natives of 

 the Pacific Islands ; also, a large Mahogany Book- 

 case. 



