435 Costa Rica Miocene — Olsson 263 



Mactra alata Spengler Plate 28, figure 3 



Mactra alata Spengler, 1802, Skriv. Naturch. Selsk., vol. 5, part 2, 



P- 99- 

 Mactra alata Reeve, 1854, Conch. Icon., Mactra pi. 8, fig. 29. 

 Mactra {Mactrella) cf. alata Maury, 1917, Bull. Amer. Pal., vol. 5, 



P- 395- 



A thin, fragile species distinguished by its sharp dorsal-pos- 

 terior angle and hatchet-shaped shell. It is related to the <f.r(?- 

 leta^ already referred to, but is a very much broader shell. In 

 exoleta, the dorsal-posterior area is flat or concave, with a slight 

 raised line which is bounded on each side b}^ a markly depressed 

 or sunken band. In alata this area is convex. 



It is living today on the Caribbean side of the Isthmus, be- 

 ing quite common on sandy beaches. 



Gatun Stage: Boucary Creek. 



Hill No. J, Banana River. 



Section HARVELLA, Gray 



Mactra estreilana, n. sp. Plate 28, figure i 



Shell rather large, ovate, thin, slightly convex; surface reg- 

 ularly, concentrically plicated; the fragmentary type specimen 

 about 38 mm in height has about 32 plicae which on the center 

 of the shell disk are spaced about 2 mm apart; these plicae com- 

 mence as in typical Harvella at the dorsal-posterior carina which 

 bounds the escutcheon and extends across the surface to the an- 

 terior extremity; the surface is, in addition, finelj^ concentrically 

 striated; lunule large elliptical, smooth or finely striated; escut- 

 cheon smooth; interior not known. 



lycngth 50, height 38, semi-diameter 8 mm. 

 Although this is not a very rare species in the Costa Rican 

 Miocene, its thin, delicate shell renders the collecting of good 

 specimens very difficult and it is usually secured only as distort- 



