ORTHOTETES 75 



specimens of Orthotetes described and illustrated by Fischer de Waldheim 

 were examples of a septate shell essentially identical as to generic char- 

 acters with Waagen's and Hall and Clarke's interpretation of Derby a. 

 For the accommodation of the nonseptate shells called Orthotetes by these 

 authors, Girty has proposed the name Schuchertella, which genus has al- 

 ready been discussed in this report. Among the septate shells two rather 

 distinct types are recognized, in one of which, the true Orthotetes, the 

 septa are joined to form a small triangularly pyramidal cavity in the ros- 

 tral portion of the pedicle valve, while in the other group, for which the 

 name Derbya is retained, no such rostral cavity is present. In the Mis- 

 sissippian faunas here considered only two species of these septate shells 

 are recognized, in both of which the rostral cavity is commonly present, 

 although it is often small and sometimes the union of the dental lamellae 

 with the septum is more or less incomplete. 



ORTHOTETES KEOKUK (Hall) 

 Plate VII, Figs. 1-4 



1849. Orthis crenistria Yandall and Shumard, Cont. to Geol. Ky., pp. 19, 



21. 



1858. Orthis keokuk Hall, Geol. Iowa, vol. 1, pt. 2, p. 640, pi. 19, figs. 5a-b. 

 1883. Streptorhynchus keokuk Hall, Rep. N. Y. State Geol. for 1882, pi. 



(11) 41, figs. 1-3. 



1890. Orthis keokuk Gordon, Am. Geol., vol. 5, p. 261, pi. 1, fig. 7. 

 1892. Derbya Keokuk Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 11, 



figs. 1-3. 



1894. Orthis keokuk Keyes, Mo. Geol. Surv., vol. 5, p. 63. 

 1899. Derbya keokuk Girty, Mon. U. S. Geol. Survey, vol. 32, p. 524. 



Description. Shell large, transversely subelliptical in outline, the 

 greatest width near the mid-length of the shell, the hinge-line shorter 

 than the greatest width, the cardinal extremities rounded or obtusely an- 

 gular. The dimensions of a nearly complete but somewhat undersized 

 specimen are : length of pedicle valve 64 mm., length of brachial valve 60 

 mm., greatest width -[-80 mm., length of hinge-line 65 mm., height of 

 cardinal area 8.7 mm., thickness of shell 25.6 mm. 



Pedicle valve nearly flat, usually slightly concave in the central portion 

 and a little elevated in the umbonal region towards the beak ; mesial sinus 

 obsolete ; cardinal area flat, of moderate height, sloping posteriorly from 

 the hinge-line to the apex at an angle of about 128 degrees to the plane of 

 the valve, the lateral margins sharply defined and angular; the delthy- 

 rium about as wide Or a little wider than high, completely closed by a 

 convex deltidium. Internally a strong median septum extends from the 

 beak anteriorly for about one-third the length of the valve ; the teeth are 

 supported by short, non-septate dental plates which are scarcely more 

 than thickened ridges upon the inner surface of the shell, one on each side 



