336 PALAEONTOLOGY OF NEW YORK. 



The cardinal area is a feature more generally developed among the forms in- 

 cluded by Waagen under his term Aphaneropegmata (=Protremata, Beecher), 

 that is, among forms possessing the deltidium, but it is very irregular in its occur- 

 rence among all the articulate Brachiopoda. The genus Spirifer furnishes a 

 most striking instance of its persistence in the deltarium-bearing shells ; its usual 

 absence in Pentamerus .and Concuiuium serves to demonstrate that it is not an 

 indispensable character of its group. It is probable that the existence of this 

 area has little fundamental connexion with the condition of the pedicle-passage. 

 It is a very palpable fact that there is a much more intimate relation 

 between it and the general form of the shell; thus in the elongate shells, like 

 the terebratuloids, meristoids, retzioids and the pentameroids for the most part, 

 there is no such area present. Where the form of the shell is more generally 

 transverse, as among the Ohtbidm, in Strophomena, Clitambonites, Derbya, 

 Spirifer, etc., the area is highly developed. This area is a characteristic 

 feature of all early deltidium-bearing species, and, where it manifests itself 

 occasionally in one of these groups which has for the most part lost, or never 

 developed this area, as in Porambonites, Gypidula and Pentamerella among 

 the pentameroids, its appearance may be regarded as the resumption of a 

 primitive or original character which was normal for that division of the 

 Articulates in some period of its history. 



Similarly we meet with a cardinal area in an early rhynchonellid type, 

 Orthorhynchula, and this is an evidence of the first significance as indicating 

 the source from which the extensive group of the Rhynchonellas originated. 

 These are shells which, at a very early period, assumed the deltarium or sec- 

 ondary condition of the pedicle-covering. It would be presumptuous to assume 

 that a single species of this great group developed a cardinal area solely from 

 mechanical causes, such as obstructed growth on the posterior margins of the 

 valves. Its presence seems, rather, to suggest the perpetuation of an ancestral 

 character indicating that these modified shells have been derived from a more 

 primitive condition in which the cardinal area was normal and, no doubt, 

 accompanied by a deltidium. In the absence of further evidence such a char- 

 acter is of much interest and importance. 



