142 Studies in Geology, No. 3 



dementia dariena Toula, Jahrb. k.k. geol. Reichs., Bd. 61, p. 725, 

 pi. 27, figs. 9, 10, 1908. 



dementia dariena Brown and Pilsbry, Proc. Acad. Nat. Sci. Phila., 

 vol. 63, p. 371, pi. 28, fig. I, 191 1. 



Shell large, inflated, ventricose, oblique. Anterior straight 

 for a short distance before the beaks, then broadly rounded, 

 descending abruptly into the ventral margin, which curves 

 broadly in sub-circular form into the posterior margin. 

 Umbones somewhat high, prosogyrate, not strongly invo- 

 lute, proximate, widely conical. Surface somewhat exca- 

 vated before the beaks and slightly produced, very convex 

 mesially, becoming more flattened ventrally. Sculpture of 

 inoceriform concentric undulations, prominent from the 

 beaks about half-way down the shell ; thence ventrally the 

 surface is nearly smooth. About 17 undulations are promi- 

 nent in the average valve. Upon these undulations are 

 superimposed fine concentric lines of growth which continue 

 on the smoother part of the shell, where they are irregular 

 in strength. The sculpture of the right and left valves from 

 Zorritos is identical. There is no lunule, but a deep 

 escutcheon provided housing for the cylindrico-ellipsiodal 

 ligament. The escutcheon, whose upper margins are sharp, 

 projecting, and rather broadly lanceolate in outline, is 

 marked on its inner surface by very fine longitudinal lines ; 

 a somewhat prominent furrow separates the main surface 

 of the escutcheon from the overhanging lip, which is 

 wide posteriorly, decreasing to nothing at the beaks. Hinge 

 as follows : "Xach vorn stehen zwei Zahne, der zweite 

 bogig gekriimmt ; hinter einer dreieckigen tiefen Grube 

 folgt dann ein lamellarer Doppelzahn, der bis an den Rand 

 des Schloszfeldes hinabreicht." — Toula, 1908. Length, 75 

 (approx.) ; height, 86; diameter, 48.5 mm. 



This form is of particular interest on account of its 

 limited known occurrence ; thus far it has been reported 

 from only Panama and Costa Rica, and it is apparently 

 absent in the Tertiary of Bowden and the Dominican Re- 



