52 PALEONTOLOGY OF CALIFOKNIA. 



very slightly produced aud pointed to one side in the axis of the 

 shorter diameter of the shell. Surface irregularly rugose, the pli- 

 cations or undulations showing a tendency to radiation, though 

 faintly. 



Longest diameter, l.G inch; shortest, 1.3 inch; height, .35 inch. 



A single specimen from the Pliocene of San Fernando, near "Wiley's. 



Although 1 have but a single specimen of this shell, and it belonging to a genus in 

 which specific characters are usually difficult to define, I teel warranted in naming 

 it on account of its marked dissimilarity from all the other known species on the 

 coast. It approaches most nearly in size Lottla gigantea, but the central and more 

 elevated apex will at once distinguish it ; from Acmcea patina, pelta and scabra, it 

 can be known by the more elliptical form, undulated surface, more elevated apex 

 and irregular outlines, while from A. persona and spectrum, its central apex, thrown 

 to one side, instead of in advance, the absence of the strongly marked regular 

 radiating ribs, and its larger size, separate it. How far the specific value of all 

 the characters given in the description will extend, cannot be definitely ascertained 

 except by an examination of more specimens. It is not improbable that the irregu- 

 lar outline and the undulating ribs on the surface may be due to a great extent to 

 the form of the surface to which the shell was attached during life. It is well 

 known that these shells, like most other forms which attach themselves to one 

 spot, often borrow the pattern of the surface on which they grow. This is often 

 shown by barnacles, cryptas, and anomias ; the present genus rarely going beyond 

 a mere distortion of its normal form, or the assumption of a few abnormal undu- 

 lations. When the surface to which the young animal fixes itself is too rough, it 

 not unfrequently smooths a spot corresponding in size with the exposed portion of 

 the animal. This I have observed in the case of Acmcca persona and A. spectrum 

 at Santa Barbara, where, growing on a coarse-grained metamorphic sandstone, 

 almost every shell fits into a little pit, barely more than a line in depth, level at 

 the bottom, and corresponding exactly to the form of the shell. 



ZIRPHtEA, Leach. 

 Z. Gabbii, Tryon. 



PI. 15, Fig. 10. 

 (Zirphma Gabbii, T., Proc. Phil. Acad., 1866, p. 144, pi. 1, fig. 1.) 



This shell was described by Mr. Tryon from a single dead valve, sent to him by 

 me from San Francisco. I obtained it from a miscellaneous collection of shells 



