188 PALAEONTOLOGY OF CALIFORNIA 



This shell, called by Mr. Conrad " the finger-post of the Eocene," proves to be 

 the "finger-post" of the Tejon Group. Like its ally of the Eocene, it is peculiar 

 to the formation, and its geographical limits are coextensive with the group. 



At the time that I described it I had a pretty large series of specimens from 

 near Fort Tejon, Martinez, and Clayton. Since then it has been found in several 

 other localities, and fine specimens have been obtained near New Idria, and west 

 of Griswold's, on the road from the latter place to San Juan. I had, however, no 

 specimens of the true planicosta for comparison, and was obliged to depend partly 

 on figures, partly on my memory, as I mentioned in the note to the description. 

 That description, should be slightly modified, so as to read — "Shell variable in 

 shape, subtriangular to subquadrate," the subquadrate forms usually being the 

 more adult, though some specimens, four and four and a half inches across, are as 

 distinctly triangular as the typical planicosta. The number of the ribs is as con- 

 stant as is usually the case in costate shells. I have before me one specimen with 

 twenty-one, another with twenty-three ribs. I have compared my specimens with 

 shells from the London Clay, and from the Alabama Eocene, and find that, except 

 in the extreme quadrate forms, they are absolutely identical in all characters save 

 one. The hinges are so similar that I despair of making an intelligible written 

 description of their minute differences, and should hardly feel willing to trust an 

 artist with their delineation. But a character exists by which the merest tyro 

 could separate the two species. It is in the shape of the cross-section of the ribs. 

 The surface is well represented by the figure in the first volume, and I have only 

 to append a series of outline cross-sections to illustrate this difference. Fig. 83 is 

 a magnified section of the ribs in their normal state; 83 a, is a section of these ribs 

 after having been corroded by atmospheric or other causes ; while 83 b is a section 

 of the ribs of C. planicosta. It will be observed that even when the ribs of C. 

 Hornil have been flattened by weathering, they are still different from the Eocene 

 shell, in their slight elevation, and in the wide interspaces. Some old specimens 

 of C. Hornii have the same peculiarity as some old specimens of C. planicosta, in 

 the ribs becoming more flat, and even nearly obsolete towards the base. This is 

 not a constant character in either species. One fine large shell of the latter form, 

 from Claiborne, Ala., in the museum of the Philadelphia Academy, has the ribs 

 as sharply defined at the base as towards the beaks. 



CLISOCOLUS, N. Gen. 



PL 30, Fig. 84. 



Shell thin, Lucinseform, without a lunule; ligament external, 

 lodged in a deep groove ; hinge edentate, having in the right 



