EUMETRIA 441 



The reference of this species to the genus Eumetria may be incorrect, 

 but there is no way to determine from the material available whether it is 

 a member of this genus or of Acwnbona. It has been referred to both 

 genera in the past, but never upon sufficiently well founded evidence. 



Horizon. Chouteau limestone. 



EUMETRIA PERSTRIALIS Rowley 

 Plate LXXVI, Figs. 4-6 



1900. Eumetria perstrialis Rowley, Am. Geol., vol. 25, p. 266, pi. 5, figs. 

 28-30. 



Description. Shell small, longer than wide, ovate-subcuneate in out- 

 line, the. greatest width anterior to the mid-length, the long postero- 

 lateral margins nearly straight and meeting at the beak in an angle of 

 about 57 degrees, the antero-lateral margins regularly rounded into the 

 less curved, subtruncate anterior margin. The dimensions of the holo- 

 type are : length of pedicle valve 10 mm., length of brachial valve 8.9 mm., 

 greatest width 7.2 mm., thickness 6 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly from the point of greatest convexity to the postero-lateral mar- 

 gins and becoming a little inflected to the cardinal extremities, curving 

 with a long, gently convex slope to the anterior margin and with a 

 shorter, more strongly convex curvature to the beak ; mesial sinus or fold 

 obsolete ; the beak rather large and prominent, projecting notably beyond 

 that of the opposite valve, pierced by a large foramen which encroaches 

 upon the umbonal region of the valve, the delthyrium closed by deltidial 

 plates; cardinal area very small, scarcely greater than the delthyrium. 



Brachial valve with its greatest convexity posterior to the middle, the 

 surface curving abruptly to the postero-lateral margins, sloping with a 

 long, gently convex curvature to the anterior margin and with a shorter 

 and more strongly convex- curvature to the beak ; mesial fold or sinus ob- 

 solete ; the beak rather strongly incurved beneath the pseudodeltidium of 

 the opposite valve. 



Surface of both valves marked by fine, simple, radiating plications, 

 three or four of which occupy the space of one millimeter at the front of 

 the shell, the entire number recognizable being about 42, in the posterior 

 half of the shell these plications become so fine that the shell appears to 

 be smooth except when examined under a lens. A little posterior to the 

 middle a rather strong line of growth marks each valve in the holotype, 

 with a somewhat smaller line between it and the umbo, other concentric 

 markings obsolete. 



Remarks. A single example of this species, the holotype, has been ex- 

 amined. Its internal characters are unknown so it is not possible to de- 

 termine whether or not it possesses the essential characters of Eumetria or 



