118 PALEONTOLOGY OF NEW YORK. 



the brachial valve contained a pair of diverging lamellae 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 lamellflB, 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 

 Rhtnohospira of the Silurian ; forms which represent different stadia in the 

 line of development of these plicated shells, 



•The determiDation of the critical features in Rbtzia, Ecmbtbia, Hdstbdia, etc., has been attended with 

 peculiar embarrassments, which patient and careful work alone have been able to successfully surmount. 

 Representatives of ail these genera are of infrequent occurrence and rarely well preserved for the study of 

 their complicated inteHors. To (he student, who chooses to follow the methods here adopted for investigat- 

 ing these fossils, it may be advantageous to know that in our first 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 Rbtzia, Hbstkdia and Acamboka, with the crescentic arms of the hinge-plale in Humktria. It is cer- 

 tainly a curious fact that two parts so distinct and capable of jiroducing similar effects in transverse 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. 



