MORPHOLOGY OF THE ORTHOID SHELL 37 



(9) The Pionodettta or Schizophoria type, in which the shaft is delicate, the myophore may or 

 may not be lobate in front, while the muscle attachment is near the dorsal portion of the myophore, 

 leaving an unoccupied portion ventrally. Here the muscle attachments are not uncommonly deeply 

 sunk so that the myophore simulates the Hebertella type. Finally, in Schizo-phoria the cardinal 

 process may be resorbed and its place occupied by low ridges (pi. A, fig. 14). 



BRACHIOPHORES OR BRACHIAL APPARATUS 

 In previous works treating of the Protremata, little attention has been paid to the calcified 

 brachial apparatus, and in consequence the classificatory value of these structures has been almost 

 completely overlooked. In the present studies we produce some new knowledge regarding these 

 parts in the most primitive families of the Protremata, and have used these structures, in combina- 

 tion with the other parts of the shells, to prove relationships of genera hitherto unsuspected. To 

 facilitate more detailed description, it is necessary to have a new terminology defining the various 

 parts of the cardinalia. 



The simple brachial apparatus of the orthids has usually been termed the crura, but it is not yet 

 established that these processes are homologous with the crura of the rhynchonellids or the "crura" 

 of the terebratulids and spirifers, which continue unbroken into the descending lamellae of the loops 

 or spires. In the usual Paleozoic rhynchonellid, the crura are long curved processes borne on the 

 inside of a socket plate or crural base. In the orthids, on the other hand, the structures commonly 

 termed the crura are also the "socket plates," since they bound the sockets. Furthermore, these so 

 called crura in many of the orthids actually have elongate processes which extend into the valve pre- 

 cisely as do the crura of rhynchonellids. In separated valves these processes are commonly broken 

 off as is also the case among the rhynchonellids, but in a few orthids the entire brachial apparatus has 

 been preserved (see pi. 5, fig. 24). 



Among the orthids, we propose to apply the term brachiofhore to the structures on either side 

 of the notothyrium which bound the sockets and to which were attached the elongate brachiofhore 

 processes; to the latter in turn was fastened the lophophore. In some genera the brachiophores are 

 supported dorsally by plates, and to these the name brachiofhore flates or suf forts is given. It will 

 be seen from the above that the brachiophore or so called crus of the orthid is evidently homologous 

 with the crural base or socket plate of the rhynchonellid, rather than with the crus of the latter. 

 Our terms are applicable only to the more primitive or orthoid stage of development of the brachia, 

 and among these shells there are several types of brachiophores as follows: 



(1) Eoorthis-Billingsella tyfe (see pi. 1, figs. 19, 25). — Since the brachiophore in these forms 

 is derived from that of the most primitive of the Cambrian orthids, it naturally should be of the 

 simplest type. The brachiophore is a flat blade with its inner margin flush with the margin of the 

 notothyrium and lying obliquely under its anterior surface. The dorsal palintrope thus overlaps the 

 posterior or outer surface of the brachiophore. The socket is the space between the sloping outer face 

 of the brachiophore and the palintrope. In the Billingsellidas, Nisusiidas, and Protorthidas the 

 sockets are narrow and slitlike, and the brachiophore is supported along its anterior surface by the 

 swelling of the notothyrial platform (pseudocruralium of Walcott) which extends laterally under the 

 anterior surface of the brachiophore. This type of brachial process may have evolved into the one 

 seen in the Orthidas (Orthinae, Hesperorthina;), Dinorthidas, and Clitambonitidas. 



(2) Orthis-Hesferorthis tyfe (see pi. 2, fig. 10; pi. 4, fig. 21; pi. 5, fig. 24).— This type is 

 not unlike the previous one but is more advanced. The brachiophores were more efficient, since they 

 had a greater antero-ventral extension. In this type the brachiophore is an elongate blade or rod, 

 occasionally grooved along the inner face as in Hesferorthis, unsupported except for the addition of 

 shell substance under the anterior face during the growth of the notothyrial platform. The brachial 

 processes of the Russian Orthis callactis-calligramma group are very close to those of Billingsella and 

 Eoorthis in their simplicity. On the other hand, the brachiophores of Hesferorthis are advanced, 

 being grooved along the inner face and in well preserved shells terminated by an elongate process 

 or point. The Orthis type of brachiophore is present in several genera in various modifications 

 brought about by deposition of adventitious material. This is best seen in Glyftorthis, in which the 

 notothyrial platform simulates that in Hebertella. 



