Clark: The San Lorenzo Series of Middle California 159 



and nearly straight; posterior dorsal slope fairly steep and slightly convex. 

 Anterior end strongly produced, rather attenuately rounded ; posterior end broadly 

 rounded. Surface smooth except for medium fine, incremental lines. Extending 

 from the beak to the ventral side of the posterior end is a well-marked ridge, 

 posterior to which the surface is depressed rather strongly. 



Dimensions. — Length, about 93 mm. ; height, about 29 mm. ; diameter of both 

 valves, about 18 mm. 



Occurrence. — University of California localities 14, 708, 1131 and 1167. 



This species is of the same general type as Spisula albaria Con- 

 rad 130 Spisula sclbycnsis Packard, 131 and Spisula ramonensis Packard. 

 The hinge plate of S. ramonensis attenuata is not known. The chief 

 basis for separating it from S. ramonensis, found in the same horizon, 

 is the more strongly- produced anterior end and the more prominent 

 posterior ridge. It appears to be a distinct form and worthy of at 

 least varietal rank. 



SPISULA? SAXADOMOIDES, n. sp. 

 Plate 16, figure 1 

 Type specimen 11268, Coll. Invert. Palae. Univ. Calif., loc. 1131 

 Shell medium in size, broadly elongate-ovate; beaks fairly prominent, strongly 

 inturned, situated about two-thirds the distance from the posterior to the anterior 

 end; anterior dorsal margin slightly excavated; anterior end rather strongly 

 produced, broadly and regularly rounded ; posterior end broadly and regularly 

 rounded ; posterior dorsal margin long, nearly straight. Ventral edge gently 

 arcuate. Surface smooth except for incremental lines; anterior and posterior 

 dorsal margins of shell rather strongly pouting, one of the common characteristics 

 of many of the species of this genus. Hinge plate of left valve only imperfectly 

 exposed, showing a small deltoid cardinal with a deep resilifer pit. 



Dimensions. — Length, 60 mm. ; height, 40 mm. ; diameter of both valves, 

 26 mm. 



Occurrence. — University of California localities 52 and 1131. 

 The outline of this species is very unique, reminding one of a Saxi- 

 domus rather than of a Spisula. 



Genus MULINIA Gray 

 MULINIA?, sp. 

 Dimensions. — Height, about 90 mm. ; length, about 93 mm. 

 A cast of a large mactroid shell was found at University of Cali- 

 fornia locality 1131. It is referred questionably to the genus Mulinia, 

 chiefly because of the thickness, outline and fairly heavy muscle 



130 For original description of Spisula alliaria Conrad, see Amer. Jour. Sci., 

 ser. 2, vol. 5, p. 432, fig. 4, 1878. 



131 For original description of Spisula selbyensis Packard, see Univ. Calif. 

 Publ., Bull. Dept. Geol., vol. 9, no. 15, pp. 292-293, 1916. 



