1916] Packard: Mesozoic and Cenozoic Mactrinae 289 



15 mm. Those of the plesiotype are : length, 74 mm., height, 53 

 mm., convexity, 15.5 mm. 



Neither the type nor any specimens that were labeled by the 

 members of the State Geological Survey are known to exist. Sev- 

 eral specimens from the general region from which the type was 

 obtained appear to satisfy the description of this species as given by 

 Gabb. One of these has been made the plesiotype of this species. 



Although the ratio of the height to the length of this species 

 is quite variable, this form is quite readily separable from other 

 known mactrine species. Certain specimens might possibly be con- 

 fused with a young specimen of S. catilliformis, but the lack of the 

 characteristic posterior flexure of that species and the widely differ- 

 ent type of dentition easily characterizes Gabb's species. 



Occurrence. — Oligocene : Agasoma gravidum zone, Mount Diablo 

 region, California. 



SPISULA ABSCISSA (Gabb) 

 Plate 23, figure 4, and plate 24, figure 2 

 Schizodesma abscissa Gabb, Pal. Cal., vol. 2, p. 20, pi. 4, figs. 34 and 

 34a, 1869. 



This species was described by Gabb as follows : 



Shell large, thick, irregularly subquadrate; beaks large, prominent, 

 pointed forwards, nearly central; anterior end obliquely truncated; base 

 broadly rounded; posterior end very strongly and abruptly truncated, the 

 truncation ending at an angular ridge which passes from the beaks to the 

 posterior basal angle. Surface marked by rough irregular lines of growth. 

 Hinge composed of large robust teeth; lateral teeth long and thick. 



A specimen obtained from the vicinity of San Pablo Bay, Cali- 

 fornia, appears to satisfy the requirements of the description of 

 this species as given by Gabb. The type being lost, a specimen 

 from University of California Locality 1608, is here taken as the 

 plesiotype. 



The hinge plate is rather broad, chondophore wide and deep, 

 slightly overhung by the arm of the cardinal ; left cardinal rather 

 small, arms of equal length, extending ventrally to about the middle 

 of the hinge plate ; laterals long and rather thin, smooth, flat-topped, 

 distant from the beak, the anterior one not reaching to the arm 

 of the cardinal. Hinge of right valve unknown. The dimensions 

 of the type as measured on the original figure are : length, 100 mm., 

 height, 98 mm., convexity, 36 mm. 



