1916] 



Packard: Mesozoic and Cenozoic Mactrinae 



299 



specimen figured in this paper are: length, 21 mm., height, 15.5 mm., 

 convexity, 5 mm. 



This species was made the basis of the genus Cymbophora, which 

 according to Dall should have only subgeneric value. This trigonal 

 species lacks the pronounced radial ribbing of the closely related 

 form S. gabbiana, with which it was confused by both Gabb and 

 Whiteaves. 



The type of this species was obtained from Pence's Ranch, Butte 

 County, California. 



Occurrence. — Cretaceous: (?) Horsetown (Diller) ; common in 

 the upper Chico of the coast region. 



SP1SULA GABBIANA (Anderson) 

 Plate 27, figure 2 



Mactra gabbiana F. M. Anderson, Proc. Cal. Acad. Sci., 3rd ser. vol. 7, 

 p. 74, pi. 7, fig. 156, 1902. 



This species was described by Anderson as follows : 



Shell moderate in size, somewhat resembling M. ashburnerii Gabb, but 

 generally with a heavier shell, and more strongly grooved concentrically; 

 umbonal angle strongly marked, especially near the base; anterior surface 

 flattened but not excavated. 



The hinge of this species is but imperfectly known. One speci- 

 men from Chico Creek, California, which undoubtedly belongs to 

 this species shows a well-developed anterior arm to the right car- 

 dinal, but no trace of a posterior arm. Another specimen of the 

 left valve has a rather shallow chondophore bounded by raised 

 margins, characteristic of the subgenus Cymbophora. The cardinal 

 is prominent, massive, extending ventrally nearly across the hinge 

 plate; the arms are very thick; anterior lateral apparently fused 

 to the dorsal side of the anterior arm of the cardinal ; posterior lat- 

 eral long, but little higher than the adjacent shell margin. Pallial 

 sinus rather shallow, broadly rounded. Its dimensions, as meas- 

 ured upon the figure of the type are: length, 44 mm., height, 33 mm. 



This species differs from S. ashburnerii in being typically much 

 larger and more heavily ribbed and less angulated posteriorly. 



The type of this species was obtained from Henley Creek, Sis- 

 kiyou County, California. 



Occurrence. — Cretaceous: Chico of Henley Creek, Chico Creek 

 and numerous other Chico localities of California. 



