746 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Subfamily LepTELLInINAE Ulrich and Cooper, 1936 
Plectambonitacea having the general appearance of Leptelloidea and with an 
elevated visceral disk in the brachial valve. When present, the cardinal process 
is a simple ridge with a lateral ridge on each side. 
Genus PETRORIA Wilson, 1926 
Plate 221, E, figures 29-36 
Petroria Witson, Geol. Surv. Canada, Bull. 44, pp. 27-20, pl. 5, figs. 15-18, 1926. 
Since its proposal this genus has not been very well known, and some ques- 
tion attends its proper age designation. The type specimens were taken from the 
Beaverfoot limestone and are associated with an equivocal assemblage. The gas- 
tropod Palliseria has proved to be identical with Mitrospira which forms a 
prominent zone in the high Pogonip of western United States. Petroria does not 
have the structure of the more advanced leptelloids to which it is related but is 
more suggestive of the Canadian genus Leptella. In accord with this view is 
the presence of Orthis marshalli Wilson and Plectorthis ? sinuatis Wilson which 
are Canadian types. Although the writer has not seen specimens of either of 
them, the first species suggests Orthambonites and the second reminds one 
strongly of Orthidiella. It seems very probable, from the above evidence, that 
Canadian and Richmond fossils have been brought together by structural 
complications. 
Through the kindness of Dr. W. A. Bell of the Canadian Geological Survey 
the writer was permitted to etch apart the valves of one of the cotypes of Petroria 
rugosa. The specimen selected for this delicate operation was G. S. C. 6754a. 
Part of the pedicle valve of this specimen had been broken off, but the brachial 
valve was nearly intact. The etching process was carried out successfully, and 
the two valves were easily separated. 
The pedicle valve has a moderately long interarea slightly overhanging the 
delthyrial cavity which is moderately deep. Dental plates are not visible. If 
they existed in the young Petroria, they have been obliterated by shell deposit in 
the umbonal and delthyrial chambers before the individual reached the size of the 
specimen under examination. The muscle area is somewhat triangular and is not 
deeply impressed. Each of the vascula media consists of 2 narrow channels sepa- 
rated by a broad fold, thus forming a double track extending from the antero- 
lateral angle of the muscle area obliquely for half the shell length. The shell is 
somewhat thickened at the anterior extremity of the pallial trunk. 
In the brachial valve the chilidial plates are prominent and almost close the 
notothyrium. The interarea is moderately long. No cardinal process is present, 
a feature suggesting Leptella and early origin. The sockets are shallow and 
formed by the sloping face of the short brachial processes. The notothyrial cavity 
is made obsolete by swelling of shell substance between the brachiophores. The 
median ridge extends to the extremity of the visceral disk and is continued from 
there to the shell margin. The visceral disk is low, not extended strongly over 
the anterior trail. The muscle field is oval and somewhat thickened. 
