118 
PALEONTOLOGY OF NEW YORK. 
the brachial valve contained a pair of diverging lamellsB which supported a con¬ 
cave plate, and that the continuation of the plate probably supported the spiral 
cones. The differentiation of the parts of the hinge-plate in Eumetria is not 
reproduced in any other genus, and the description here given is derived from 
excellent preparations of the interior of the valve, the element of possible error, 
which may exist in determinations where the nature of the material has ren¬ 
dered necessary reconstructions from serial sections, being here eliminated.* 
The species Retzia vera. Hall, from the Kaskasia limestone of the lower Car¬ 
boniferous series, was figured in illustration of the various distinctive features 
of the shells for which the term Eumetria was proposed, and it may be regarded 
as the typical form of the genus; the Retzia serpentina, de Koninck, from the 
Carboniferous limestone of Belgium, was also, at that time, regarded as a 
typical representative of the same group, but we have not a sufficiently 
critical knowledge of its interior characters to feel assured that the species is 
congeneric with R. vera. 
The loop in Eumetria shows an interesting variation in form ; in its posterior 
position, anterior direction, long, reflected stem and slight terminal bifurcation, 
it suggests the structure found in the Carboniferous species, Athyris Dawsoni; 
at the same time the absence of a saddle and the mere inception of the acces¬ 
sory lamellae, occurring in association with the elongate-ovate shell, indicate 
the structural relationship to Meristina, Merista and Meristella. Its nearest 
allies, however, in this and other respects, are the genera Hustedia of the Coal 
Measures, Acambona of the Burlington limestone, Retzia of the Devonian and 
Rhynchospira of the Silurian ; forms which represent different stadia in the 
line of development of these plicated shells. 
*The determination of the critical features in Retzia, Eumetbia, Hustedia, etc., has been attended with 
peculiar embarrassments, which patient and careful work alone have been able to successfully surmount. 
Representatives of all these genera are of infrequent occurrence and rarely well preserved for the study of 
their complicated intei'iors. To the student, who chooses to follow the methods here adopted for investigat¬ 
ing these fossils, it may be advantageous to know that in ourfii-st determination we fell into the very natural 
error of identifying the testaceous crescent, or split tube, which appears in sections of the umbonal cavity 
of Retzia, Hustedia and Acambona, with the crescentic arms of the hinge-plate in Eumetria. It is cer¬ 
tainly a curious fact that two parts so distinct and capable of producing similar eflects in ti'ansverse sections, 
should occur in the umbonal cavity of these fossils, but we are now satisfied that they do not coexist in any 
of the genera, or at least that where the crescent of the hinge-plate is at its maximum, the foraminal tube 
has a minimum development. 
