74 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Perhaps the most striking variation within the 

 Orthidas is that of Productorthis, which externally sim- 

 ulates the contour, profile, and sculpture of certain 

 genera in the Productidas but never bears the promi- 

 nent tubular spines of the latter. 



terior and lateral margins of the ovarian impressions. 

 The subparallel pallial sinuses are homologous with the 

 widely divergent marks of the Billingsellida:, and orig- 

 inate at the front of the diductor scars as in that 

 family. The septum separating the two sinuses may 



Table 5 



Mainly American 



Mainly European 



Glyptorthin^ 



ORTHINi€ 



Hesperorthin^ 



Cyrtonotella 



Ptychopleurella 



Eridorthis 



Paurorthis 



PRODUCTORTHINiE 



Productorthis 



I 



TAFFIINiE 



Panderina 



ANGUSTICARDINIINiE 



' EOORTHID;^ 



In the ventral valve the muscle-scars are usually 

 confined to the delthyrial cavity. The diductor im- 

 pressions may or may not be prolonged in front of the 

 adductor track, but they never swing around in front 

 to enclose the diductors as in the Dinorthidas. Adjus- 

 tor scars are usually not clearly defined but are fairly 

 definite in some of the younger genera. Occupying 

 the umbo-lateral spaces and produced nearly to the 

 front and lateral margins are subreniform ovarian 

 impressions. These are bounded on the inside and 

 antero-median margins by prominent pallial sinuses. 

 The latter take their origin at the front ends of the 

 diductor tracks, as is usual in orthoid brachiopods, ex- 

 tend forward nearly parallel, and are separated by a 

 very narrow ridge or low septum only. At the front 

 of the valve each sinus swings abruptly laterally and 

 extends for a longer or shorter distance along the an- 



be broad or wide and is forked at the front or not, 

 depending on the age of the shell or how deeply 

 impressed the sinuses are at that point. 



In the dorsal valve the brachiophores are usually 

 simple, rodlike, or narrow and bladelike; they are 

 closely related to those of the Billingsellidas but appear 

 to be more highly specialized. Support of the brach- 

 iophores is accomplished by deposition of adventitious 

 shell substance under their antero-dorsal surface and 

 upon the notothyrial platform. The cardinal process 

 is always a simple septum which may or may not bear 

 the diductor scars; impressions of these are frequently 

 to be seen on the notothyrial platform on either side 

 of the cardinal process at its base. Dorsal pallial im- 

 pressions are too infrequently seen to serve importantly 

 in any family definition. 



Derivation out of the Billingsellidae is demonstrable 



