342 
PALEONTOLOGY OF NEW YORK. 
eral, erect or convex growths upon the margins of the delthyriura, which may 
be interpreted either as remnants of a resorbed convex deltidium, or as highly 
accelerated secondary deltaria. Every now and then specimens will show a 
clearly developed cardinal area; always in Stricklandtnia, frequently and nor¬ 
mally in Gypidula, rarely and of exceptional occurrence in Pentamerella. 
Stricklandinia possesses so straight and long a hinge, so sharply defined an 
area and so short a spondylium, that it is more natural to regard this genus as 
the accompaniment, rather than the close organic kin of the other pentameroids, 
deriving its difFerentials directly from those long-hinged and straight-hinged 
shells of the early Silurian, which constitute the genus Syntrophia. 
It will not now appear a matter of inexplicable aberrancy that the spondylium 
presents itself in the great secondary groups comprising the rhynchonellids, 
and those shells with calcified brachidia. Hence we meet with it in Cyrtina 
and Camarospira in a highly developed state, and in Camarotgechia in a less 
advanced condition, while Amphigenia presents the remarkable combination of 
a spondylium coexistent with a shell of completely Rensselaerioid aspect (that 
is in respect to form, contour, muscular markings gnd articulating apparatus), 
an 1 with rhynchonelloid brachial supports. 
Attention has already been directed to the fact that some of the 
RiiYKcnosELiAVJE, early in their history, occasionally retain a well-defined car¬ 
dinal area, and that, in default of other evidence, the presence of this char¬ 
acter may be regarded as indicative of the common origin of Orthis, the 
SmopnoMENiDjE, and the Rhynchonellas. The earliest phyletic stages of the 
rhynchonellids must have been highly accelerated, for there is no evidence of 
any form which has shown the slightest trace of a deltidium. Nevertheless the 
early forms of the Silurian, sach as Orthorhynchula and Protorhyncha, rarely 
show any indication of deltaria at maturity but the delthyrium, in its final 
stage, is unobstructed and simple, as in young conditions of later rhynchonellids 
in which the deltaria fully develop. We may look upon the RHYErcno^ELLiDAn 
as a family whose characters became established very early and have been per¬ 
petuated up to the present without departure, at any time, from the early 
derived type. 
