28 H. Doc. 129. 



(2.) OSTREA. A small parasitic Oyster, on twigs, probably of G-or- 

 gonia, usually projecting equally to either side. The form is ellipti- 

 cal, the under valve having a groove for the twig, with a corresponding 

 eminence in the upper valve ; sometimes it adheres by but one edge, 

 and then assumes an oblique, alate form, like Avicula. It ^is quite 

 thin, lineated, radiately ornamented with fascicles of brown lines. 



Locality. San Diego. W. P. Blake. 



Another oyster from San Diego, about two inches long, narrow and 

 moderately widening, slightly sigmoid in form, its margins simple and 

 the extremity somewhat truncate and undulate or dentate, with by 

 four or five radiating grooves; shell compact, upper valve purplish. 

 It occurs in clusters intimately grouped, much like the parasitic 

 oysters of Carolina, but more elongate, less degitate at the end, less 

 cavernous under the beak, and less black within. 



Locality. San Diego. IV. P. Blake. 



(3.) PECTEN. A large turgid species, like P. gibbus or purpuratus, 

 and no doubt described. It has about twenty ribs, which are square 

 and a little broader than the interspaces, which are scaly. 



Locality. San Diego. W. P. Blake. 



ARTICLE III. 



NOTES UPON SILICIFIED PLANTS FOUND FOSSIL IN CALIFORNIA BY 

 W. P. BLAKE, GEOLOGIST OF THE EXPEDITION UNDER THE COM- 

 MAND OF LIEUTENANT R. S. WILLIAMSON. 



Among the boulders of syenite, greenstone, and similar rocks piled 

 in confusion along the banks of Kern river, a short distance below the 

 ford ; I found a mass of yellow rock, siliceous and jasper y in its char- 

 acter, that was filled with silicified fossil stems, traversing it in every 

 direction. These stems were so completely preserved that every cell 

 and tube was distinctly visible, and the cross fracture showed the or- 

 ganization beautifully. The diameter of the stems is about half of 

 an inch, and they vary in length from one to three inches and more ; 

 but they could not be detached from the rock except in fragments. 



The boulder containing these beautiful fossils was about 18 inches 

 in diameter, and numerous specimens were procured from it. It must 

 have been transported from the head waters of the river, or from the 

 sedimentary beds a short distance above, having been broken out b} r 

 the undermining action of the stream. It is very desirable that the 

 original locality of these fossils should be discovered and their geolo- 

 gical associations made known. 



It is probable that the examination of the rock in situ would de- 

 velop many interesting forms not found in these transported masses. 



Similar fossils were afterwards found "in place" near the summit 

 of the Sierra Nevada, [in the pass called the Canada de las Uvas. They 

 were on the eastern slope, and, therefore, within the limits of the 



