SCHIZOPHORIA 167 



crowded anteriorly where they are sometimes subimbricating. The shell 

 substance is pierced by radiating canals or tubules which open at intervals 

 along the summits of the costae. 



Remarks. This species has commonly been included in 8. swallovi, but 

 besides occurring at a lower horizon it possesses well-marked distinguish- 

 ing characters. It never attains so great a size as S. swallovi, its pedicle 

 valve is never so flat, it possesses a distinct mesial sinus toward the front 

 of the pedicle valve, and instead of a slight mesial depression towards 

 the front of the brachial valve there is a tendency towards a slight ele- 

 vation or fold. In all these characters the species approaches the De- 

 vonion 8. striatula, and it is essentially a form intermediate in character 

 between this species and 8. swullovi. 



Horizon. Fern Glen formation. 



SCHIZOPHORIA SWALLOVI (Hall) 

 Plate XXII, Figs. 1-6 



1848. Orthis resupinata Christy, Letters on Geology, pi. 3, figs. 1-2. 

 1858. Orthis swallovi Hall, Geol. Iowa, vol. 1, pt. 2, p. 597, pi. 12, figs. 



5a-b. 

 1883. Orthis Swallovi Hall, Rep. N. Y. State Geol. for 1882, pi. (6) 36, 



figs. 23, 24. 

 1892. Orthis Swallovi Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 6, 



figs. 23, 24. 

 1894. Orthis swallovi Keyes, Mo. Geol. Surv., vol. 5, p. 63, pi. 38, fig. 5. 



Description. Shell large,, resupinate, broader than long, transversely 

 subelliptical in outline, often slightly emarginate in front, the hinge-line 

 about one-half or a little more than one-half the greatest width, the car- 

 dinal extremities rounded. The dimensions of two pedicle valves are: 

 length 41 mm. and 38.5 mm., width 51.7 mm. and 49 mm., length of hinge- 

 line 31 mm. and 23 mm., height of cardinal area 6 mm. and 4 mm., convex- 

 ity 9.2 mm. and 7.5 mm. The dimensions of a brachial valve are : length 

 39.6 mm., width 46.2 mm., convexity 14.3 mm. 



Pedicle valve nearly flat or only slightly convex over the greater por- 

 tion of its surface, most prominent in the umbonal region, curving some- 

 what abruptly to the cardinal margin, a little compressed towards the 

 cardinal extremities; the median portion of the valve usually undifferen- 

 tiated in the posterior half of the valve and becoming slightly depressed 

 towards the front in a broad, shallow and undefined sinus, occasionally 

 this sinus originates in the umbonal region, and sometimes it is slightly 

 more depressed at either side towards the front with the median portion 

 a little higher; beak rather small, only moderately produced beyond the 

 hinge-line posteriorly ; cardinal area rather small, concave, with the curv- 

 ature increasing towards the beak, broadly triangular, the lateral mar- 

 gins well defined and usually slightly concave in passing from the beak 



