Atrypae of the Central Alisscu.i Devonilcn 3 
Stropheodonta inflexa Swallow, Leiorhynchus sp. nov. and the 
rare crinoid—Dactylocrinus concavus (Rowley). 
Branson’ figures A. rockfordensis in his Devonian of Mis- 
souri, but refers the species to A. hystrix Hall and its strati- 
graphic position in the Snyder Creek is not given. 
Average measurements of a series of the best preserved 
material in the collection give the following: length 23.7 mm., 
breadth 27mm., thickness (of conjoined valves) 11.3 mm., num- 
ber of striae in space of 5 mm. 2—3. 
ATRYPA CALLAWAYENSIS SP. NOV. 
Plate 3, figs,9, 10. 11. 
Shell longitudinally subovate in outline, gibbous, strongly in- 
equivalve, hinge line short, cardinal extremities rounded, greatest 
breadth of valves just formward of the posterior third of the 
shell. 
Pedicle valve the smaller, convex in the umbonal region but 
deeply sinuate in the anterior portion. 
Brachial valve convex in the posterior portion but developing 
a medial fold which becomes quite prominent in the anterior 
one third, an upward flexing of the lateral margin of the valve, 
gives a somewhat winged expression to the shell in some speci- 
mens. 
The exterior of both valves is covered with fine radiating 
striae four or five to the space of 5 mm. Striae are increased, 
both by implantation and bifurcation. A few ill defined con- 
centric lines of growth appear at intervals on the valves but 
8towth varices are only feebly developed. Structurally the shells 
are very thin; undistorted examples with conjoined valves are 
rare. Measurements of ten specimens average as follows {length 
26 mm., breadth 26 mm., thickness 13 mm., number, of striae in 
the space of 5 mm. 5. 
This species is found only in the lower third of the Snyder 
Creek Shale, —Strophonella zone—where it is associated with 
7Missouri Geol. Survey, Vol. 17, Series 2, pl. 20, figs. 9, 10, 1, 
and pl. 23, fig. 7. 
