26 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



angular umbonal ridge extending near and almost parallel with the dorsal 

 margin from the beak to the posterior end ; surface marked by regular, 

 comparatively rather coarse, concentric lines and furrows ; hinge and other 

 internal features unknown. 



Length of the best preserved specimen 28 mm.; height 16 mm.; con- 

 vexity of single valve about 5 mm. Another fragment indicates a speci- 

 men with corresponding measurements about one third greater. 



The material is too imperfect and fragmentary for a satisfactory specific 

 description or figure and as the hinge is unknown no definite generic ref- 

 erence can be made. The form, sculpture, and other features observed in- 

 dicates that it belongs to the Mactridae though probably not to Mactra in 

 the restricted sense as defined by Dall. It has a greater resemblance to 

 Spisula. 



Locality and position. From the Ammonite (Belgrano) beds ten miles 

 east of Lake Pueyrrydon. 



CORBULA CRASSITELLOIDES Sp. nOV. 

 PI. IV, Figs. 1 4 and 15. 



Shell small, subtriangular, rather convex ; beaks prominent, broad, sit- 

 uated slightly in advance of the middle ; dorsal margin descending almost 

 equally forward and backward from the beaks ; ventral margin convex ; 

 anterior end broadly rounded ; posterior end obliquely subtruncate ; um- 

 bonal ridge distinct, angular, descending obliquely to the postero-ventral 

 angle ; surface marked by regular fine concentric lines. 



Length of an average specimen, 5.5 mm.; height, 4 mm.; convexity of 

 the two valves, 4 mm. 



This abundant little species resembles C. bodenbenderi Behrendsen 1 

 from the Neocomian of Arroyo Triuguico, Argentine Republic. It is 

 somewhat smaller than Behrendsen's species, more inequilateral, more 

 convex, and the posterior end is relatively broader. In general form it 

 resembles the young of some species of Crassatella. 



Locality and position. Represented by numerous specimens from the 

 Ammonite (Belgrano) beds ten miles east of Lake Pueyrrydon and at 

 mouth of canon four miles east of the same lake. 



'Zeitschr. deutsch. geol. Gesellschaft, Bd. 44, 1892, p. 19, pi. 3, figs. 6 a -6 d . 



