342 PALEONTOLOGY OF NEW YORK. 



eral, erect or coavex growths upon the marghis of the delthyriiira, 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 Stricklandinia, 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 tlie 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 comjirising the rhynchonellids, 

 and those shells with calcified brachidia. Hence we meet with it in Cvrtina 

 and Camarospira in a highly developed state, and in Camarotcechia in a less 

 advanced condition, while Amphioenia presents the re:narkable combination of 

 a spondylium coexistent with a shell of completely Rensselaerioid aspect (that 

 is in respect to form, contour, muscular markings and articulating apparatus), 

 an 1 with rhynchonelloid brachial supports. 



Attention has already been directed to the fact that some of the 

 RnTNcnoNELLiDJi, carly 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 

 Stropbomenid^e, 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 Orthorhvnchula 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 RarNcnoyELLiD.E 

 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. 



