377 



Costa Rica Miocene — Olsson 



205 



Gatun Stage: Port Limon. 



Coll. 5, Red Cliff Creek 



Genus AMUSIUM, Bolton 

 Amusium luna Brown and Pilsbry Plate 17, figure t 



Amusium Mortoni Bose, 1906, Boll, de Tnst. Geol. de Mexico, Numero 

 22, p. 24, pi. I, figs. 3, 6, 7, 9. Not Amusium Mortoni Ravenal, 

 1844, 



Amusium Mortoni Bose, op. cit, p. 74, pi. 8, figs, i, 2; pi. 3, fig. 3. 



Peden {Amusium) cf. Mortoni Toula, 1908, Jahrb. der K-K Geol. 

 Reichsanstalt, Wien, vol. 58. p. 714, pi. 26, figs. 8, 9. 



Peden {Amusium) luna Brown and Pilsbry, 19x2, Proc. Acad. Nat, 

 Sci. Phila., vol. 64, p. 514, pi. 23. fig. i. 



This is the common Gatun Amnsium in Panama and Costa 

 Rica. Its valves are nearly equal and but slightly convex; ears 

 equal and small, and defined from the rest of the shell by a 

 sharp ledge; the umbos, as well as the rest of the shell disk are 

 smooth, except for the faint lines of growth; the internal ribs 

 are in pairs, numbering about 23 and spaced at intervals nearly 

 twice the width of the pairs of ribs themselves. 



Bose and Toula both referred this species to the Upper 

 Chesapeake Miocene and recent Amusium Mortoni Ravenel, 

 from which it is very distinct. Amusium Mortoni is not only 

 much larger (height 170 mm), but is porportionately much 

 broader, and with larger ears which are defined simply by a line 

 from the rest of the shell. 



The Dominican A. papyraceum Gabb has larger ears, the in- 

 ternal ribs are more crowded and the left valve is generally 

 strongly flexed along the anterior and posterior submargins. 

 Amusium Toulce Brown and Pilsbry, is found in the Gatun of 

 the Canal Zone. Its surface is marked with dark-colored rays 

 and is said to contain no internal ribs. 



