PLATE I, XXII— Continued. 



Fig. 21. Dorsal view of a somewliat elongate shell, willi full, pi'oininent uml)0, and destitute of siirfafe 

 plications. 



Uji])!']' Devonian. Lime Civtk, lowri. 

 Figs. 22, 28. Front and profile views of a moi'e orbicular shell, als( without iilications. Should the smooth 

 .shells, now included undei- this name, be found to pie.sent i)ernianeut differences from the pli- 

 cated forms, they may be termed Gypidnta lOfvU. 

 Upper Devonian. Independence, lovM. 

 Fig. 24. The interior of the umbonal poi-tion of conjoined valves ; showing, above, the free sponilyliiun of 

 the pedicle-valve, and, beneath, the sessile spondylium of the br.achial valve. 



Siliciotis beds. j)i'(il)ably of the :ige (»f the HaniiltOTi group, frntntf Indiana. 



GyPIDULA LjEVIUSCULA, Hilll. 



Figs. 25, 26. Dorsal and profile views of an avei-age specimen ; showing the smooth, evenly convex valves 

 and the almost complete obsolesence of fold and sinus. 

 Upper Devonian. TAine C'rei:k, hiwa,. 



Gypidula Romingeki, sp. UOV. 



Fig. 27. Cardinal view of a pedicle-valve ; showing a well defined cardinal area, the teeth and the form of 

 the spon(iylium. 



Fig. 28. Dorsal view of the undjonal region of the siie<'imen represented in fig. 30, efilarged to show the 

 sharply defined cardinal area and the deltidial plates or remnants of the deltidinm. X 2. 



Fig. 29. The interior of a bi-achial valve j showing the curved walls of the sessile spondylium. 



Fig. 30. Dors.al view of an individa.al of average size, with an unusually flat brachial valve ; .showing the 

 cardinal area, jilates of the deltidium and coiu'se, irregulai'ly iluplicate plication of the surface. 



Figs. 31, 32. Dorsal and profile views of a large specimen with a finer and more regular surface plication, 

 more conspicuous and deeply incurved ventral umbo and clearly denned sinus on the brachial 

 valve. The shell is largely exfoliated over the Ijrachial valve, exposing the broken base of the 

 spondylium and the genital markings about it on the surface of the cast. 



Fig. 33. The inteiior of a large brachial valve ; showing the form and extent of the spondylium. 

 Hamilton group. Near Alpena, Michigan. 



