﻿NO. 2 FOSSILS FROM PANAMA AND COSTA RICA DALL 9 



ARCA (NOETIA) MACDONALDI, new species 

 Shell rather large when fully grown, but mostly represented by 

 half grown specimens, elevated, subtriangular, with very prominent 

 prosocoelous beaks, the anterior end obliquely truncate ; posterior end 

 rounded bluntly, anterior end extended into a point near the base. 

 The part of the shell posterior to the truncation sculptured with 

 about twenty-five broad, flat, similar radial ribs with very narrow, 

 shallow, distally channelled interspaces, crossed by fine, equidistant, 

 wavy, concentric lines ; carina of the truncation rounded, truncate 

 area with about fourteen squarish, smaller ribs with narrower inter- 

 spaces which frequently carry one or two fine radial threads. The 

 beaks are much twisted forward, the ligamentary area is entirely 

 behind them and transversely, closely grooved ; in front of the beaks, 

 the valve is deeply impressed forming a sort of cavity not sharply 

 bounded ; ligamentary area narrow, inclined at a high angle to the 

 hinge-line ; posterior teeth with the last 7-9 angular as in Yoldia, 

 succeeded by numerous very closely crowded flat teeth, the series 

 divided under the apex of the beak; anterior series shorter, flat, 

 crowded, numbering 9-15 according to the age of the shell. Interior 

 of the shell smooth with well marked muscular impressions, the 

 lateral and basal margins strongly plicate in harmony with the exter- 

 nal sculpture. Valves subequal, the beaks nearly medial. Height 45 ; 

 basal length 54; length of hinge-line 31 ; max. diameter 48 mm. 

 Station 5882 m. Types, U. S. Nat. Mus. 214344. 

 This is an abundant and characteristic shell of the Costa Rica 

 beds, collected by Pittier and MacDonald. It belongs to the group 

 of A. trinitaria Guppy, and several others of the region. 



ARCA (SCAPHARCA) PITTIERI, new species 



Shell of small size, inflated, oblique, with prominent, slightly 

 prosocoelous beaks and a wide, short, lozenge-shaped, mostly smooth, 

 amphidetic area between them ; anterior end attenuated and produced 

 but not sharply truncate, posterior end much shorter and very 

 bluntly rounded; exterior with about twenty-six radial ribs sculp- 

 tured with squarish nodules, and separated by shallow, slightly 

 narrower interspaces crossed by fine concentric striations. The 

 valves are nearly equal and similarly sculptured, with the beaks 

 nearly at the posterior fourth of the length. Internal margins 

 strongly, radially channelled in harmony with the external ribs ; 

 hinge-line straight, the tooth series divided somewhat in front of the 

 beaks with about 22 anterior and 18 posterior, closely adjacent. 



