PLATE XXVI. 



(Dgore* 1-7, S-U by R. P. Whitfield; 8 by F. It. Meek; IS by B. Emmons.) 



Lejjeml : A. OaiMinal area. T'. Internal cast of tube. 



^. Delthyrium. j. Canliiial i)roce88. 



DfC. Deltidial g-i-ooves. b. Dental sockets. 



b'. Tranoverse dellhyrial plate. r. Didnctor scars. 



T. Tubular ptirtion of plate. a. Adductor scars. 



Genus CYRTIA, Dalman. 



Pago 40. 



Cyrtia alta, Hall. 



Tig. 1. The internal cast of the brachial valve ; showing the striated cardinal area, structure of the artic- 

 ulating apparatus and the faint plications on the median fold. 



Tig_ 2. The front view of an internal cast of the pedicle-valve ; showing the muscular area and the faintly 

 plicated sinus. 



Fig. 3. An exterior view of the cardinal area of the pedicle valve ; showing the concave plate filling the 

 upjier part of the delthyrium. 



Fig. 4. A cardinal view of an internal cast of the pedicle-valve ; showing the impression left by the 

 thickened inner wall of the delthyrial plate. 



Fig. B. A portion of an internal cast of a small brachial valve retaining traces of the muscular scars. 

 Chemung group. MeadvUle, Penn. 



Genus SYRINGOTHYRIS, A. Winchell. 



Page 47. 



Syringothyris typa, a. Winchell. 



Fig. 6. The apical portion of a pedicle- valve ; showing the dental plates, the delthyrial plate, with the 



central tubular portion projecting beyond its bi-oken margin. 

 Fig. 7. The opposite side of the same specimen ; showing the extension of the canaliferous tube which is 



slit along its inner surface. 

 Fig. 10. The interior of the umbonal portion of the pedicle-valve ; showing the greatly thickened tubular 



or canaliferous plate, not closed along its inner surface. 

 Burlington limestone. Burlington, Iowa. 



Syringothyris subcuspidatus, Hall. 



Pig. 8. A cardinal view of the original sjiecimen. 



Keokuk limestone. Keokuk, Iowa. 

 Fig. 11. The interior of a large pedicle-valve ; showing the transverse plate and the extension of the tube. 



Keokuk group. Nauvoo, Illinois. 



Syringothyris texta, Hall. 



Fig. 9. Central cardinal portion of an internal 'cast of the conjoined valves ; showing the impressions of 

 the cardinal process, dental plates and the filling of the tubular portion of the transveree plate. 



Fig. 12. A preparation, showing the elongate spiral brachial supports, the jugal processes and a portion 

 of the crura, (c.) 



Keokuk group. New Providence, Indiana. 



