EUMETRIA 437 



Genus EUMETRIA Hall 



Description. Shells small or of medium size, subovate in outline, the 

 hinge-line very short, the valves subequally convex, marked by fine, 

 simple, rounded or subangular radiating plications. Shell structure very 

 minutely and closely punctate. Pedicle valve with a prominent, incurved 

 umbo, perforated at its apex by a large, subcireular foramen which is in 

 contact with the delthyrium only at its apex; the cardinal area small, 

 but sharply defined, its lateral margins abruptly elevated above the sur- 

 face of the valve on either side, the delthyrium occupying the greater part 

 of the area, closed by the pseudodeltidium. Internally the hinge-teeth 

 are of moderate size, not supported by dental lamellae, the muscular 

 impressions inconspicuous, being only rarely sufficiently impressed to 

 leave any suggestion whatever of their presence. Brachial valve with 

 the cardinal extremities usually compressed to form minute auriculations ; 

 internally the hinge-plate is a complicated structure consisting of a pos- 

 terior, and an anterior part, extending in opposite directions from the 

 central portion, which joins the two socket walls, the posterior part con- 

 sists of a crescentic process whose horns make nearly a semi-circle and 

 extend backward into the umbonal region of the pedicle valve, the concave 

 margin of the crescent lying against the inner surface of the pseudodel- 

 tidium of the opposite valve, the tips of the crescent raised away from 

 the inner surface of the shell. The anterior part of the hinge-plate is 

 connected with the median part by a narrow, thickened, median band ; 

 it consists of a long, narrow, concave or spoon-shaped central process 

 with sharply elevated edges, these marginal ridges being produced an- 

 teriorly into the two elongate, slender, and nearly vertical crura, the 

 whole being supported by two slightly divergent, upright lamellae which 

 extend to the floor of the valve. The spiral cones approximate, with 

 eight or nine volutions, the apices directed laterally, the jugum posterior 

 in position, regularly curved from the points of origin on the primary 

 lamellae anteriorly and towards the interior, from its central point a 

 process, bifid at the extremity, extends towards and nearly to the ex- 

 tremities of the crura. No muscular markings have been observed in 

 this valve. 



Remarks. The true relations of this genus to Acambona have not been 

 cleared up, and because of the rarity of examples of the genotype of 

 Acambona, it may be a long time before the relationships of the two genera 

 can be determined. There are no characters present in the holotype of 

 Acambona prima^ the genotype of that genus, which can exclude the shell 

 i'rom Eumetria. 



