262 



Bulletin 39 



434 



The Siliqua subcBqualis Gabb, from the Miocene of Santo 

 Domingo is doubtless a Tagelus, belonging to the Section Mes- 

 opleura Conrad. It differs from the present shell in being thin- 

 ner, more depressed and more pointed posteriorly. Gabb's species 

 approaches very closely the recent T. divisus Spengler of the 

 West Indian fauna, judging from the specimens in the Maury 

 collection at Cornell, differing mainly in its larger size. 



The interior of the Costa Rican shell is concealed so that the 

 presence or absence of an internal rib cannot be determined. 

 Its heavier shell and general form, is more that of typical Tage- 

 lus differing from the recent gibbus Spengler in its more central 

 beaks. 



Gatu7i Stage: Rio Betey. 



Genus MACTRA (L.) Lamarck 



Subgenus MACTRELLA, Gray 



Mactra exoleta Gray Plate 28, figure 2 



Madra exoleta Gray, 1837, Mag. Nat. Hist., p. 372. 

 Mactra exoleta Reeve, 1854, Conch. Icon., Mactra, fig. 16. 



Comparision of our fragmentary shell with recent examples 

 from the Bay of Panama show no inportant differences. The 

 species is notable for its high, narrow, inflated umbos, strongly 

 depressed posterior-dorsal margin and alation of its dorsal-pos- 

 terior angle. 



At the present time, this species is restricted to the Pacific 

 side, but represented on the Caribbean coast by the next, the 

 Mactra alata Spengler, common along sandy beaches. It is 

 therefore of more than usual interest to find these two species, 

 together in the Miocene beds of the Caribbean slope of Costa 

 Rica. 



Gatun Stage: Boucary creek. 



