178 



Bulletin 39 



350 



G. acuticostata Sowerby of the Santo Dominican Miocene. They 

 vary somewhat in size, (20 to 32 ram in height), are usually de- 

 pressed and with an outline suggestive of the recent East Coast 

 and Upper Miocene G. pecfinata Gmelin. Typical acuticostata 

 does not seem to occur in Costa Rica and I have adopted Brown 

 and Pilsbry name of canalis. 



The G. canalis Brown and Pilsbry from the Canal Zone, and 

 the trilobicostata Pilsbry and Brown from Colombia, are both bas- 

 ed on young shells 15 mm or less in height. Both forms can be 

 duplicated amongst the Costa Rican shells and are but varieties 

 of the same species. 



In its usual and more abundant form, the shell is depressed, 

 with numerous (about 26), closely spaced rounded or angled ribs. 

 These ribs are overrun by even, close, concentric threads. They 

 represent the usual canalis or its variet}^ trilobicostata, and aver- 

 age in height about 25 mm. More rarely, the shells are larger 

 and as illustrated by figure 4 and 5 the ribs may become divided 

 into 2 or 3, strong, radial cords. The following measurements 

 will show the usual range in size. 



Height 22, diameter 21, thickness 5.5 



24 23 5.5 



32 32 8 



29 29 8 



Gaiun Stage: Gatun, C. Z. 



Comadre; Ba?iana River; Coll. / East Grape 



Point Creek. 

 Old Man Sam Creek, C. R. 



Glycymeris jamaicensis Dall Plate 19, figures 3, 4 



Pectunculus pennaceus Guppy, 1866, Quart. Journ. GeoL Soc. London, 

 voL 22, p, 293. Not of Lamarck. 



Axinea pennacea Gabb, 1873, Trans. Amer. PhiL Soc, voL 15, p. 255. 



Pectunculus decussatus Guppy, 1876, Qart. Journ. Geol, Soc. London, 

 voL 32, p. 532. Not of Linne. 



