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89 ' 



TERTIARY FAUNA OF FLORIDA 



separated by a delicate but distinct lamella walling off the ligament from the 

 pit. M. alabamicnsis " Lea," which Conrad referred at one time to S. modicella 

 and another to his M. prcFtcitnis, appears nowhere in Dr. Lea's writings, and 

 there seems to be no evidence that such a name was ever proposed by him. 



Subgenus MACTROTOMA Dall. 



Section Microiiiactra Dall. 



Mactra (Mactrotoma) cymata n. s. 



Plate 33, Figure 23. 



Oligocene marl of Oak Grove, Florida ; Burns. 



Shell small, thin, with prominent undulated beaks ; subequilateral, 

 rounded in front, rather pointed behind, the base moderately arcuate; sur- 

 face sculptured with fine incremental lines, the umbones with ten or more 

 distinct concentric ripples ; the posterior slope moderately angulated or 

 carinate anteriorly; pallial sinus rather short, rounded. Lon. 31.5, alt. 20, 

 diam. 10 mm. 



This species much resembles the Pliocene M. iindiila, and differs from it 

 chiefly in being smaller, more triangular, and more pointed behind. 



Mactra (Mactrotoma) undula n. s. 

 Plate 28, Figure 12. 



Pliocene of Darlington, South Carolina, Burns ; of the Caloosahatchie 

 River and Shell Creek, Florida, Dall and Willcox. 



Shell small, moderately thick, ovate-oblong, externally nearly smooth or 

 marked with feeble concentric lines of growth and, near the ventral edge, with 

 fine, somewhat irregular wrinkles ; beaks small, adjacent, not prominent, con- 

 centrically undulated with rounded ripples, becoming rapidly obsolete but 

 varying in strength in different individuals ; dorsal areas smoother, impressed, 

 long and narrow, with a single narrow, slightly elevated line extending from 

 the beak to the posterior ventral margin in each valve; posterior extreme 

 usually rounded, more slender than the anterior, or slightly rostrate ; anterior 

 end somewhat shorter than the posterior, rounding into the arcuate base; 

 pallial sinus rounded in front, not quite reaching the vertical from the beaks ; 

 muscular impressions large; hinge well developed, normal to the subgenus. 

 Lon. 42, alt. 26, diam. 14 mm. 



This interesting species is closely allied to the recent M. californica Con- 

 rad of the Californian coast, and is one of the rather numerous instances 



