BracuioropA. 923 
deltaria. Every now and then specimens will show a clearly- 
developed cardinal area; always in Srrickianprnta, frequently 
and normally in Gypipoxa, rarely in PenrampREeLta. Srrick- 
LANDINIA 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 and straight hinged shells of the early 
Silurian, which constitute the genus SyyTropHia. 
It will not now appear a matter of inexplicable aberrancy that 
the spondylium presents itself in the great secondary groups com- 
prising the rhynchonellids, and those shells with calcified 
brachidia. Hence we meet with it in Cyrtiva and Camarosprra 
in a highly-developed state, and in Camaroiaonta in a less 
advanced condition, while AmpsicEen a presents the remarkable 
combination of a spondylium coexistent with a sheli of completely 
rensseleroid aspect (that is, in respect to form, contour, muscular 
markings and articulating apparatus) and with rhynchonelloid 
brachial supports. 
Attention has already been directed to the fact that some of 
the RuYNcHONELLID», early in their history, occasionally 
retain a well-defined cardinal area and that, in default of other 
evidence, the presence of this character may be regarded as 
indicative of the common origin of Ortuis, the S7ropxHo- 
MeENID#z, 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 deltidium. Nevertheless the early forms 
of the Silurian, such as OxtsoraynosuLta and  Prorto- 
RHyscHa, rarely show any indication of deltaria at matu- 
rity, but the delthryium, 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 Rarn- 
CHONELLID# as a family whose characters became established very 
early and have been perpetuated up to the present without wide 
departure, at any time, from the early derived type. 
In the study of the multifold variations of the articulates bear- 
ing calcified spiral brachial supports, the Heticopremara of 
Waagen (1883), the conclusion has enforced itself that the degree 
175 
