62 PALAEONTOLOGY OF CALIFOENIA. 



when I obtained specimens from Estrella, whence Mr. Conrad described his 

 " Balanus Estrellanus," and later, Mr. Gibbes, a surveyor, presented me with a re- 

 markably fine example from the hills north of Buena Vista Lake. The original 

 specimen, figured in Vol. 6, Pacific K. E. Eep., came from the Santa Margarita 

 Eanch, northeast of San Luis Obispo. These localities serve to fix its geological 

 age beyond question as belonging in the Bituminous Shale, the best-marked mem- 

 ber of our Upper Miocene. This being ascertained, the next point is to determine 

 what are its relations. 



To better understand the shell, it may be well to premise by remarking that Mr. 

 Conrad mistook the two ends of his specimen, and called the base, the aperture. 

 The following description, in the absence of more than one species, will have to 

 serve for both generic and specific purposes, since in so exceptional a case it is diffi- 

 cult to determine which of the minor characters are of only specific value. 



Shell (? lower valve) elongated, cup-shaped, growing in clusters, cemented 

 throughout the entire length wherever in contact; cross-sections irregularly 

 rounded or angulated; lower end of each shell curved outwards, to accommodate 

 the increased diameter of its neighbors ; walls of the cup composed of (two, or) 

 three layers. (I have not been able to determine finally, whether an outer imper- 

 vious layer exists, though I think I have detected it in the Estrella specimens.) 

 The mass of the wall is composed of a series of sub-parallel, longitudinal hollow 

 tubes, rudely and irregularly divided by minute septa; inner layer entire, homo- 

 geneous, and thin. The interior contains a " body chamber," or large unoccupied 

 space, at its widest end, below which the cavity is filled up by an indefinite number 

 of very irregular septa, undulated, and presenting a mammillated surface upwards. 

 These septa are produced by lateral prolongations of the inner wall, the "funnel- 

 shaped tubes," mentioned in Mr. Conrad's diagnosis, being the portions last formed 

 as the mantle was retracted. Some of these "tubes " reach to the surface of the 

 preceding septum, while many of them are merely little stalactite-like points, and 

 it is doubtful whether any of them are perforate. In the specimen from which 

 the first description was taken, these chambers and even the tubes of the outer 

 walls are all hollow. In the others they are filled with crystalline carbonate of 

 lime; and in these latter, the outer, or " body chambers," arc filled with an indu- 

 rated sand. The outer surface is rudely and longitudinally striate ; the striae are 

 sometimes crossed by abrupt transverse breaks or subsquamose lines. 



The thickness of the outer wall is a quarter of an inch or less, the tubules com- 

 posing it being about .04 inch in diameter; the inner stratum of the wall is 

 about .01 to .02 inch in thickness. 



I have used the term "lower valve," because I believe this to be a bivalve shell, 

 for the reasons given below. The upper valve, if it existed, has never yet been 

 detected ; but, if my view is correct, this need not be wondered at, especially since 

 fossils are rare in the deposit in which the shell belongs ; and with all the facilities 



