BRACHIOPODA. 99 



In the separated valves, the hingeline is often nearly straight, the muscular area of 

 the ventral valve is short and broad, the length from the apex being about equal to the 

 width. There is a slight thickening of the shell at the base of the rostral cavity. The 

 surface around the muscular area is papillose, and limited by a thickened border, except 

 in front, where it is discontinued. Fine vascular markings are sometimes visible near 

 the margin. In the dorsal valve there is a thickened septum in the upper part of the 

 muscular area. 



The spires of full-grown individuals show about fifteen turns in each." 



Remarks — Specimens from the Snyder Creek have usually been identified as A. 

 hystrix var. occidentalis Hall by Missouri collectors. They differ from that species in 

 their larger number of plications, 17 to 25 as compared to 8 to 14, in the plications 

 remaining distinct to the margin, and in the brachial valve being less gibbous. No 

 specimens with spines are in the collections examined. On plate 23, figure 7, the Atrypa 

 is inserted for comparison with typical A . spinosa from the Snyder Creek shale. 



Occurrence — Rare in the Snyder Creek shale of Montgomery County. 



Family Spiriferidae 

 Genus Spirifer Sowerby 



Spirifer annae Swallow 



Plate 19, figures 13, 15-17; plate 24, figure 17 



1860. Spirifer annae Swallow, Trans. St. Louis Acad. Sci., I, p. 641. 

 1860. Spirifer amarus Swallow, Trans. St. Louis Acad. Sci., I, p. 642. 

 1897. Spirifer annae Schuchert, Bull. U. S. Geol. Surv., 87, p. 381. 

 1897. Spirifer amarus Schuchert, Bull. U. S. Geol. Surv., 87, p. 381. 

 1909. Spirifer amarus Greger, Am. Jour. Sci., XXVII, p. 377. 



Swallow's description — "Shell very small, sub-semipyramidal. Ventral valve 

 semi-pyramidal; slightly arched from the beak to the anterior and lateral margins; 

 area high, arched, triangular; sides shorter than the base on the cardinal line; beak 

 small, pointed, and slightly incurved; mesial sinus simple, well defined, concave, or 

 flattened at the bottom, foramen narrow. Dorsal valve semicircular, slightly convex, 

 flattened towards the cardinal extremities; mesial fold sharply defined, depressed, simple 

 beak small, slightly incurved over the cardinal line; cardinal line as long or shorter than 

 the width of the shell. Surface marked with from six to ten plications on each side of 

 the mesial sinus and fold, and by numerous concentric imbricating lamellae. 



Length, .25; breadth, .33; thickness, .20." 



Revised description — Shell very small for the genus, angular, semiconical, wider 

 than long, the greatest width usually along the hingeline. Resembles Cyrtina in shape 

 and dimensions. Usual dimensions: Length 5 to 6 mm., breadth 6 to 7 mm., thickness 

 3 to 4 mm. The largest specimen observed measured: Length 11 mm., width 15 mm., 

 height, 9 mm. Shells 2/3 as large as the largest given are not uncommon. 



Pedicle valve very convex and Cyrtina-like in shape; beak small, pointed, slightly 

 to strongly incurved; cardinal area very high, slightly to quite strongly arched, lateral 

 margins sharply defined; lateral slopes each marked by 5 to 8 simple rounded plica- 

 tions; mesial sinus shallow, rounded at the bottom, originating at the beak .and in- 

 creasing gradually in width to the front margin. 



Brachial valve slightly convex, with the greatest convexity near the back; mesial 

 fold low, not rising above the rest of the shell, separated from the lateral plications by 

 a slightly deeper furrow than those between the other plications, an incipient groove 

 near the front on the larger specimen; plications similar in number and shape to those 

 on the other valve. 



The surface is marked by numerous imbricating lines of growth. Very fine radiating 

 striae are the most striking peculiarity of the shell surface. 



