286 University of California Publications in Geology [Vol. 9 



the Fernando formation from the Aleatraz asphalt mine near 

 Sisquoc, Santa Barbara County, California. It is evident from the 

 original description that this variety is based solely upon the general 

 shape, for it is stated that the "hinge and the interior" are "quite 

 similar to the typical form". It appears to differ from the typical 

 species in its more central beaks, more depressed anterior margin 

 and more truncated anterior extremity, but similar differences in 

 shape are known to occur within the limits of the typical species 

 and such characters are there considered as having little taxonomic 

 significance. 



This species has been confused with the lutrarine species Schiz- 

 othaerus nuttallii, but it differs markedly from that species in the 

 character of the siphonal gap and in the type of the hinge. It 

 might be confused with 8. mercedensis, which is a more tumid form, 

 lacking the pronounced posterior flexure and having the anterior 

 lateral distinctly separated from the arm of the cardinal. 



The type of this species was described from a Recent specimen 

 obtained from Panama. 



Occurrence. — Recent : Straits of Juan de Fuca to San Diego, Cali- 

 fornia (Dall). Pleistocene: San Pedro, San Diego (Arnold). Plio- 

 cene: Merced group. Miocene: San Pablo group; Temblor (J. P. 

 Smith). 



SP1SULA MERCEDENSIS, n. sp. 

 Plate 13 



Shell large, ventricose, inequilateral, the beaks being consider- 

 ably anterior to the middle of the shell; anterior dorsal edge short, 

 slightly concave, with a faint suggestion of a lunule ; posterior dorsal 

 edge gently convex, curving regularly to the rounded extremity; 

 anterior end evenly rounded; base broadly arcuate; umbones small, 

 sharply pointed; an indistinct posterior flexure extends from the 

 umbones to the base, much as in Spisula catilliformis Conrad. Sur- 

 face roughened by numerous irregular lines of growth. Hinge plate 

 wide, chondophore shallow, wide, overhung but slightly by the pos- 

 terior arm of the cardinal ; left cardinal large, arms of equal length, 

 high; anterior lateral short, situated dorsal to the anterior arm of 

 the cardinal; posterior lateral high and acutely pointed. Hinge of 

 the right valve unknown. Pallial sinus not observed. The dimen- 

 sions of the type are: length 124 mm., height 93 mm., convexity 

 32 mm. 



