Clark: The San Lorenzo Series of Middle California 125 



same beds; however, as more material was obtained it became evident 

 that it was a distinct species. The main distinction between the two 

 is the great difference in size and in proportion. 



Genus MALLETIA Desm. 

 MALLETIA PACKARDI, n. sp. 

 Plate 12, figure 3; plate 14, figures 5 and 6 

 Type specimen 11154, Coll. Invert. Palae. Univ. Calif., loc. 2033 

 Shell rather small to medium in size ; beaks anterior to the middle, opistho- 

 gyrous. Anterior dorsal slope straight ; posterior dorsal slope rather strongly 

 excavated; anterior end regularly rounded; posterior end regularly rounded but 

 narrower than anterior end, the ventral margin sloping up more obliquely to it 

 than to the anterior end. Surface smooth except for somewhat irregular incre- 

 mental lines. As shown oil one of the specimens obtained from the diatomaceous 

 shale of the Markley formation (fig. 6), the escutcheon is well defined, long and 

 narrow and depressed almost at right angles to the main outer surface of the 

 shell. Lunule apparently absent. No well-defined chondrophore. About eighteen 

 taxidont teeth posterior and about twenty-one anterior to the beak, the posterior 

 and anterior rows of teeth meeting at the apex of the beaks. 

 Dimensions. — Length, 17.5 mm.; height, 10 mm. 



Occurrence. — Found at University of California localities 2035, 3055 and 3080. 

 Species of the genus Mallet ia are rare on the West Coast. Ralph 

 Arnold's species, Malletia chehalisensis, 96 from the San Lorenzo 

 formation, is apparently a Leda; as figured by him, the type (his 

 fig. 9) shows a well-defined chondrophore. One of the characteristics 

 of the genus Malletia, according to Eastman's translation of Zittel's 

 Textbook of Palaeontology, vol. 1, is that the chondrophore is lacking. 



Genus YOLDIA Miiller 

 YOLDIA COOPERI TENUISSIMA, n. var. 

 Plate 11, figure 10; plate 12, figures 8 and 14 

 Type specimen 11110, Coll. Invert. Palae. Univ. Calif., loc. 798 



This variety differs from the species Y. cooperi Gabb principally in that the 

 distance from the beak to the anterior end is shorter in proportion to the length 

 of the shell; also there is a less number of teeth in the hinge plate; in other 

 respects the species and the variety are very similar. This difference, however, 

 appears to be constant and when better and more specimens of the variety are 

 obtained it may be found that the variety should be listed as a distinct species. 



Dimensions. — Length, about r 44 mm.; height, 17.75 mm.; greatest height 

 posterior to the beak, 19 mm. 



Occurrence. — University of California localities 798, 1131, 1312, 197, 199, etc. 

 Type from locality 798 ; paratypes from localities 2754 and 1131. 



98 Arnold, Ealph, New Cretaceous and Tertiary fossils from the Santa Cruz 

 Mountains, California, Proc. U. S. Nat. Mus., vol. 34, p. 335, figs. 9, 9a, 1908. 



