PLATB lAXII-ConHnued. 



Fig. 21. Dorsal view of a somewhat elongate Hhell, with full, prominent umbo, and destitute of surface 

 plications. 



Upper Devonian. Lime Creek, Jtnea. 

 ngB. 33, 23. Front and profile \-iew8 of a more orbicular shell, alar, without plications. Should the smooth 

 shells, now included under this name, be found to pi eseut permanent differences from the pli- 

 cated forms, they may be tenned Oypidula Iwvis. 

 Upper Devonian, fndepeiuleiire, Itnoa. 

 Vig. 24. The interior of the umlmnal )>ortion of conjoined valves ; showinjf, above, the free spondylium of 

 the i)e<licle-valve, and, beneath, the sessile spondylium (if the V)rachial valve. 



Silirious IkhIb, probably of the age of the Hamilton group. Central Jv/liana. 



Gypiuula l.*;viu8Cula, Hull. 



PigfL 2S, 36. Dorsal and profile views of an avenige specimen ; showing the smooth, evenly convex valves 

 anil the almost complete obsolesence of fold and sinus. 

 Upper Devonian. lArne Creek, Iowa. 



GyPIDULA RoMINGEKI, S|). IIOV. 



V\g. 37. Cardinal view of a pedicle-valve ; showing a well defined cardinal area, the teeth and the form of 

 the six)ndylium. 



Fiy. 38. Dorsal view of the umljonal region of the specimen represented in fig. 30, enlarged to show the 

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



Pig. 39. The interior of a brachial valve ; showing the curv'ed walls of the sessile spondylium. 



Fig. 30. Dorsal view of an individual of average size, with an unusually flat brachial valve ; showing the 

 cardinal area, j>!ates of the deltidium and coiu-se, irregularly duplicate plication of the surface. 



Figs. 31, 33. Dorsal and profile views of a large sjxjcimen with a finer and more regular surface plication, 

 more conspicuous an<l deeply incurve<l ventral umbo and clearly defined sinus on the brachial 

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

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



¥ig. 83. The interior of a large brachial valve ; showing the form and extent of the spondylium. 

 Hamilton group. Near Alperia, Michigan. 



