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University of California Publications in Geology [Vol. 8 



satisfactory. The difficulty has been due partly to the lack of thorough 

 collecting, and in part to insufficient understanding of collections 

 already obtained. During the early part of 1911 an investigation 

 was undertaken by the writer with a view of working out the faunal 

 relations of these formations more fully than had been done previously. 

 The series taken into consideration included the Merced Series 

 typically developed at Seven Mile Beach, south of San Francisco, the 

 Purisima at Halfmoon Bay, San Mateo County, and the Wildcat 

 Series in Humboldt County. During the progress of this work a 

 number of undescribed species of marine mollusks were discovered, 

 and it seems desirable to present descriptions of these species in ad- 

 vance of a final paper dealing largely with the correlation problem. 

 This arrangement Avas considered desirable, as the description of 

 species is in part a matter of biology and the information will be of 

 service to workers not especially concerned with stratigraphic prob- 

 lems. Nearly all of the species here discussed were obtained from the 

 Wildcat Series. These beds have a wide distribution in the south- 

 central part of Humboldt County, California, and on account of their 

 isolation have received less attention than those farther south. 



For detailed descriptions of the localities from which the specimens 

 described and figured in this paper were obtained see page 195. 



In arranging the species the writer has followed the classification 

 used by Dr. W. H. Dall 1 in his publication on the Miocene of Astoria 

 and Coos Bay, Oregon. 



Genus Modiolus Lamarck 

 MODIOLUS ST ALDER I, n. sp. 

 Plate 22, figures 6a and 6b 

 Shell small, thin, subelliptical, sculptured with numerous radiating 

 ridges which are crossed by very fine concentric lines, producing a fine 

 tessellated surface; valves nearly equal, very inequilateral; umbones 

 conspicuous, almost terminal; umbonal ridge prominent anteriorly, 

 broadening and less prominent posteriorly ; hinge line nearly straight, 

 about one-fourth the length of the shell; upper posterior margin 

 broadly arcuate ; posterior extremity sharply and evenly rounded ; 

 ventral margin nearly straight, slightly concave at the middle in some 

 specimens ; anterior end blunt, excavated in front of the beaks. 



i Dall, W. H., U. S. G. S. Professional Paper No. 59, 1909. 



