52 JURASSIC RHYNCHONELLIDS 
the deltidial plates have not been satisfactorily observed. The smooth area on the 
brachial valve is restricted to not more than the posterior quarter of the valve. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
1°43 cm I-Io cm I-60 cm 
I-44 cm I-12 cm 1-67 cm 
Internal characters. The preservation of the available specimens is very poor and 
it was found that the internal features of the first specimen were completely oblitera- 
ted by partial silicification. A second specimen was sectioned and the structures, 
Fic. 20. Transverse serial sections of Thurmannella acuticosta sp.nov. Oxfordian. Lower 
Calcareous Grit. Filey Brigg, Yorks. (x5). 
