148 The American Geologist. ^^'^''^'i' io*^i- 
Figs. 9, 10, II. Three views of a specimen 2 mm. broad. 
Figs. 12, 13, 14. Specimen 2.5 mm. broad. 
Figs. 15 and 16. Ventral and dorsal interiors of two mature spec- 
imens. 
DESCRIPTION OF SHELL. 
Shell semi-ovate to stibquadrate in old individuals ; hinge- 
line usually less than the greatest width of the shell, especially 
in young individuals ; cardinal extremities forming an obtuse, 
or sometimes a right angle with the lateral margins. Surface 
firmly plicated ; plications increasing toward the margins by 
interstitial implantation. Crests of the plications crenulated by 
numerous equally spaced fine concentric lines. 
Ventral valve concave, with a pronounced tendency to ir- 
regular growth about the beak. In mature individuals the 
beak becomes strongly retrorse and greatly elevated, equalling 
in higlit one-half the length of the shell. Area well defined, 
flat, showing in well preserved specimens a low ridge on each 
side of the prominent deltidium and parallel with its margins. 
The younger specimens sometimes show a perforation of the 
apex of the deltidium. 
Dorsal valve regularly convex, greatest elevation about 
one-third of the way from the beak to the front margin, though 
there is considerable variation in this respect in individuals of 
different age. Usually some flattening at the cardinal extrem- 
ities. Area very narrow or scarcely at all conspicuous. 
Interior of ventral valve showing rather prominent teeth 
which diverge widely. Cardinal process in the dorsal valve 
elevated, projecting somewhat beyond the hinge-line ; notch 
shallow, the grooves on the posterior faces of the apophyses 
very faint. 
Ratio of breadth to length of an average adult individual 
about as eleven to eight. 
Obsern'ation. This form cannot be referred to the 0. 
(Terebratiilites) nnihraculnin of Schlotheim", from which it 
differs in the less length of the hinge-line, fewer plications, 
greater proportionate elevation of the ventral beak which in 
the present species becomes strongly retrorse. and the sub- 
* ScHi,OTHEiM, Petrefk, I. 256, 11.67; Schmer, Brachiop. Dek Eifel. 
216, t. 38, fis. 2; t. 4-4, fig. 4- ; Bro.vn Leth.ea, Geoo. I, 361. 
J 
