34 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



MORPHOLOGY OF THE DORSAL INTERIOR 



NOTOTHYRIAL PLATFORM 



The interarea of the dorsal valve in most genera of the Orthoidea is centrally cleft by a tri- 

 angular opening called the notothyrium, which lies opposite the delthyrium of the ventral valve j both 

 have been described in a previous section of this memoir. Beneath the margins of the notothyrium 

 is a cavity, the notothyrial cavity, which is deep or shallow depending on the degree to which the 

 floor has been built up by deposition of shell substance. The floor of this cavity is commonly built 

 ventrally from the floor of the valve by the accretion of testaceous substance, producing a swelling 

 within the walls of the cavity. To this swelling the term "platform" has recently been applied"* but 

 that term has already been used for the elevated muscular platform of the Trimerellida; (see p. 9). 

 To avoid using this term for two very diff^erent str uctures we here suggest the designation notothyrial 

 platform. 



DENTICLES AND SOCKETS 



Denticles. — Teeth for the main articulation of the valves occur only in the ventral shell, but 

 accessory processes fitting into the sockets in the posterior face of the teeth of the ventral valve occur 

 in most of the genera. These are here called denticles. They are usually wider than long and are 

 located on the outside wall of the socket. This accessory apparatus makes for rigidity in articulation 

 (t. fig. 2). 



Sockets. — The socket which receives the tooth of the ventral valve in articulation is always 

 located between the inner shell wall and the outer face of the brachiophore. The simplest type of 

 socket occurs in the Billingsellidas, Orthidas, and Dalmanellidas, in which it is merely the cavity 

 between the brachiophores and the inner wall of the valve. This may or may not be slightly exca- 

 vated beneath the palintrope, or into adventitious shell matter laid on the wall of the valve. 



A second type of socket is that which is found in the Plectorthidas, Wattsellidas, and Schizopho- 

 riida:. In these families the socket is defined by a small concave plate (fulcral plate) joining the 

 brachiophore to the wall of the valve (see pi. 22, fig. 25). 



CARDINALIA 



The articulating apparatus, the brachial supports, and the cardinal process are together known 

 as the cardinalia. Until recently this structural ensemble has been little used in the construction of 

 genera and families. Hall and Clarke in their study of Orthis gave little attention to the brachial 

 supports and cardinal process, and accordingly did not see the significant differences between Hebert- 

 ella, Dinorthisy Orthis, Plectorthis, and Dalmanella, differences pointed out in the present work. 

 The cardinalia are intimately bound up with the brachia and the musculature of the brachiopod and 

 consequently are important structures. 



CARDINAL PROCESS 



Nothing in the nature of a cardinal process is ever seen in the inarticulate orders Atremata and 

 Neotremata. Such first appears among the Orthacea of the Cambrian, but at no time in this period 

 can it be said to be strongly developed, and even in certain of the primitive genera it is a linear ridge 

 that may be absent in some species of the same genus. In Kutorgina no cardinal process is known, 

 and one may be present or absent in Nisusia and Jamesella. In the late Cambrian a cardinal process 

 is of sporadic occurrence in nearly all of the genera."^ 



The cardinal process is located on the notothyrial platform in the mid-line of the shell. It may 

 be a simple, linear, more or less thickened ridge, or it may be a far more complex structure and then 



^^ Bancroft, B. B., Mem. and Proc. Manchester Lit. and Philos. Soc, vol. 72, 1928, p. 175. 

 "Walcott, Camb. Brach., 1912, p. 306. 



