132 Carl O. Dunbar, 



lar plications separated by angular grooves of equal width, while 

 the inner part of each lateral slope is marked by four or five 

 obscure plications, of which the outer is the most indistinct. 

 Beyond these, the lateral slopes are smooth. The spondylium in 

 the ventral valve is similar to that of G. coeyinanensis, but con- 

 siderable variation may be noted in the development of the 

 median septum which supports it. In some specimens the 

 septum is strong and in a few entirely absent. 



Dimensions : Length, 40 mm. ; width, 40 mm. ; thickness, 

 about 25 mm. 



Discussion: The species is readily distinguished from all other 

 described ones by the large number and slenderness of its plica- 

 tions. It is most closely related to G. coeyinanensis, but that 

 species has fewer and much coarser plications and reaches its 

 greatest width nearer its mid-length. 



Occurrence: Birdsong shale, at numerous localities in Benton 

 and Decatur counties, especially along Birdsong Creek, at Perry- 

 ville, etc, 



FAMILY RHYNCHONELLID^ 



Rhynchotreta insinuata, n. sp. 

 Plate III, figs. I, 2 



Description: Shell small, subtriangular, width and length 

 equal. Posterolateral sides angulated and vertical, slightly con- 

 cave, meeting at the beak with an angle of less than 90°. Beak 

 pointed and erect. Valves subequally convex, without either 

 fold or sinus, flattened in the central portion, angulated at the 

 posterolateral edge and abruptly truncated around the anterior 

 margin. Thickness of the shell about equal from umbo to lateral 

 or front margin. The dorsal valve bears nine and the ventral 

 ten small rounded plications, separated by wider, concave inter- 

 spaces. Plications mostly simple, reaching to the beak, but two 

 are added by bifurcation on the dorsal valve and one by inter- 

 calation on the ventral. 



Dimensions: Length, 10 mm. ; width, 10 mm. ; depth, 5 mm. 



Discussion: The interior of this little shell is unknown, but 

 its vertical posterior sides and erect pointed beak suggest its 

 reference to Rhynchotreta. It most closely resembles R. trans- 

 versa of the Coeymans formation of New Jersey, but is larger 



