362 Report of the State Geologist. 



teriorly the median fold becomes prominent. The hinge-plate 

 consists of two flat processes, inclined toward each other but not 

 meeting. A stout median ridge supports this plate and divides 

 the muscular area. Spirals are present but their direction is 

 undetermined. 



Clinton group. . Drift Uoch in Western New YorK 



Zygospira putilla. 

 Plate 9, figs. 31, 32. 



Zygospira putilla, Hall. Palaeontology of N. Y., vol. VIII, pt. 2, p. 365, 

 pi. liv, figs. 35-37. 1894. 



Shell small, elongate-suboval in outline. Pedicle-valve the 

 more convex; umbo narrowed, apex acute, delthyrium unclosed. 

 Mediall}^ this valve is elevated by a strong double plication, the 

 parts of which diverge anteriorly, leaving a flat, low depression 

 between them, and in this lies a single faint plication. The 

 lateral slopes are considerably depressed, and each bears from 

 four to seven coarse, often irregular plications, only a part of 

 them reaching the beak. 



The brachial valve is depressed convex, with a conspicuous 

 median fold, grooved longitudinally and bounded by deep mar- 

 ginal depressions. The lateral slopes are more convex than on the 

 other valve, but are similarly plicated. 



Surface of the valves usually without concentric growth-lines. 



An average example has a length of 8 mm. and a greatest 

 ividth of 7 mm. 

 • Hudson River group. Near Edgewood^ Pike county^ Missouri. 



Atrypina Clintoni. 



Plate9, figs. 27-30. 

 Atrypina Clintoni, Hall. Palaeontology of N. Y., vol. VIII, pt. 2, p. 162, 

 pi. liii, figs. 7, 17-19. 1894. 



Shell small, subplano-convex. The pedicle-valve projecting 

 slightly at the apex, with short oblique cardinal margins and 

 nearly circular periphery. This valve has a low median sinus 

 bearing a median plication, and four or flve somewhat stronger 

 plications on each lateral slope. The brachial valve is slightly 

 convex in the umbonal region, and has but a very faintly developed 

 median fold. The surface of both valves bears rather distant 



