110 
PALEONTOLOGY OF NEW YORK. 
The external surface is radially plicate, the plications being simple. In young 
shells there is a median sinus on both valves, but as growth advances, that of 
the brachial valve develops into a low fold. Both fold and sinus bear a num¬ 
ber of small, intercalary plications, much finer than those adjoining on each 
side. Shell-substance rather sparsely punctate. 
Type, Waldheimia formosa, Hall. Lower Helderberg group. 
Observations. This generic division was separated from Retzia at a time when 
certain Carboniferous species with subalate cardinal extremities were regarded 
as typical of that genus. Subsequently these later species were found to differ 
from the strict Devonian type of Retzia, and a distinctive name, Eumetria, 
was proposed for them in 1864. In consequence of this, and while the typical 
Retzia, R. Adrieni, was less accurately understood than now, the term Rhyn- 
CHOSPiRA fell into quite general disuse, its species being commonly referred to 
the old genus, Retzia. Evidence has already been given demonstrating the 
peculiar distinctive value of Retzia as based on its typical species, and though 
there is close external resemblance between the Devonian species, R. Adrieni, 
and the earlier typical forms of the genus Rhynchospira, there is no longer 
any justification for associating the two in one division. 
There are some important features which these two groups possess in com¬ 
mon, and which, indeed, may be shared to a greater or less degree by all retzioid 
genera. In exterior structure, the finer division of the median plications, the 
smooth, gradually sloping umbo-lateral areas, occur in both Retzia and Rhyncho¬ 
spira. The coalescence of the deltidial plates is a feature occurring throughout 
the retzioid genera, though the union is perhaps more completely effected and 
subject to less variation in the Carboniferous forms. Retzia and Rhynchospira 
have a similar structure in the hinge-plates; and a character which occurs 
persistently in these genera and also in Trematospira and Parazyga, is the 
narrowing of the lateral branches of the loop just above their points of origin 
on the primary lamellae. Retzia, however, possesses a split deltidial tube in 
the umbonal cavity, which is wanting in Rhynchospira, and also a bifurcate 
termination of the stem of the loop. 
