98 THE DEVONIAN OF MISSOURI. 



1915. Atrypa reticularis Butts, Kentucky Geol. Survey, ser. 4, III, pt. 2, p. 115, pi. 

 39, figs. 1-3. 



Hall's description — "Shell depressed, suborbicular in its young state, becoming 

 gibbous and sinuate in its mature condition; hingeline often nearly straight and almost 

 equaling the width of the shell; valves nearly equally convex in the young state, the 

 dorsal valve becoming more gibbous as the shell advanced in age, and sometimes ac- 

 quiring an undefined mesial lobe down the center. The ventral valve in the young state 

 has the beak nearly straight and perforate at the apex, becoming incurved and finally 

 closely bent over the beak of the opposite valve; a narrow false area is sometimes observ- 

 able. Shell broadly and deeply sinuate in front." 



Remarks — Atrypa reticularis is the most abundant form in the Callaway and 

 Snyder Creek, and is highly varied, ranging from a small form with fine plications which 

 Rowley has called A. gregeri Rowley, to large forms with coarse plications. The A. 

 gregeri Rowley have about 4 plications to the millimeter, while the large forms may 

 measure more than one millimeter per plication. Specimens 45 mm. wide are not un- 

 usual, though at some horizons specimens as large as 25 mm. wide are rarely found. 

 Near the top of the Snyder Creek shale large specimens are about the only ones found 

 and these have seven or more concentric ridges which are strongly spinose, giving to 

 the individuals a very rough appearance. 



Near the top of the Callaway one member is largely made up of medium sized 

 A. reticularis and more than 1,000 fairly good specimens have been counted from 1/100 

 of a cubic yard of weathered limestone of this member. 



Occurrence — Abundant in Callaway and Snyder Creek, common in Mineola, rare 

 in Cooper. 



Atrypa spinosa Hall 



Plate 20, figures 8-11; plate 23, figures 8-9. 



1897. Atrypa spinosa Schuchert, Bull. U. S. Geol. Surv., 87, p. 156. 



1901. Atrypa spinosa Kindle, Indiana Dept. Geol. Nat. Res., 25th Annual Report, 



p 599, pi. 6, fig. 11. 

 1909. Atrypa hystrix occidentalis Greger, Am. Jour. Sci., XXVII, p. 376. 

 1909. Atrypa spinosa Grabau and Shimer, North American Index Fossils, I, p. 311, 



fig. 392d. 

 1913. Atrypa spinosa Clarke and Swartz, Maryland Geol. Surv., Middle and Upper 

 Devonian, pp. 587-589, pi. 55, figs. 12, 13. 



Hall's description — "Shell robust, suborbicular or ovoid; width greater or less than 

 the length; radiatingly costate and concentrically lamellose or spinose; hingeline often 

 nearly straight, a little less than the width of the shell. 



Ventral valve depressed-convex, becoming more convex in the upper part; nearly 

 flat and often a little concave towards the lateral margins, and cardinal extremities de- 

 pressed or broadly sinuate in front; beak abruptly rounded; apex truncate and perforate, 

 closely appressed and overlapping the umbo of the opposite valve. 



Dorsal valve convex, becoming gibbous in old shells, flattened or slightly concave 

 towards the cardinal angles, regularly curving to the sides and basolateral margins, and 

 a little elevated in front, but without any distinct mesial fold. 



Surface marked by strong rounded radiating costae bifurcating at unequal intervals, 

 which are much stronger in the middle of the valve, and become obsolete or appear as 

 gentle undulations toward the cardinal angles. In the middle of the valves there are 

 about seven or eight of these costae in the space of half an inch. The shell is also marked 

 by strong concentric lamellae, which are often about a line apart. In perfect shells these 

 lamellae, at the crossings of the costae, are often produced into tubular spines, which, 

 when worn off, leave the ordinary lamellose surface. The spaces between these projecting 

 lamellae are marked by fine thread-like striae. 



