PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 747 
Petroria is very much like Leptella, but a few distinctions between the genera 
can be readily seen. The known specimens of Leptella do not show the promi- 
nent, wavy lamellae of the exterior of Petroria. Inside the pedicle valve Leptella 
is provided with a callosity at the anterior end of the muscle area which is lack- 
ing in the Beaverfoot genus. The median septum of Petroria is more prominent 
and is extended anterior to the anterior margin of the visceral disk unlike that 
of Petroria. Neither genus is provided with a cardinal process. This feature 
characterizes many of the early brachiopods and suggests an early Ordovician 
rather than late Ordovician age for Petroria. Although the two genera are basi- 
cally similar, the differences mentioned are sufficient to distinguish them. 
Genus LEPTELLINA Ulrich and Cooper, 1936 
Leptellina Utricu and Cooper, Journ. Paleont., vol. 10, No. 7, p. 626, 1936; Geol. Soc. Amer. 
Special Pap. 13, p. 190, 1938. 
LEPTELLINA ABBREVIATA Cooper, new species 
Plate 191, C, figures 23-31 
Shell of about average size for the genus, slightly wider than long; greatest 
width at the hinge line. Cardinal extremities rounded, nearly a right angle. Lat- 
eral margins nearly straight, anterior margin narrowly rounded. Surface multi- 
costellate, marked by about 19 thin, highly elevated, strong costellae of which 
about 5 reach the beak; spaces between larger costellae occupied by very fine 
costellae about 8 in 1 mm. on the brachial valve where they are best preserved. 
In addition to the radial ornamentation most of valves covered by fine, closely 
spaced concentric lamellae, usually stripped away from some part of the shell. 
Pedicle valve strongly convex in lateral profile and with the greatest convexity 
at about the middle; posterior third somewhat flattened. Anterior profile still 
more convex than the other ; the greatest convexity is at the middle, but the sides 
slope steeply to the margins. Umbonal region somewhat flattened; midregion 
swollen, and with concave posterlateral slopes to the cardinal extremities. All 
slopes including the posterior slope steep. Interarea long, slightly curved, 
anacline. 
Brachial valve moderately concave with the greatest concavity developed in 
the median region. Umbo slightly concave; posterolateral areas flattened ; lat- 
eral and anterior portions moderately steeply folded in the direction of the 
brachial valve. Interarea nearly as long as the pedicle one; hypercline. Chilidium 
small. 
Interior: Pedicle muscle field pentagonal with large, widely divergent adjustor 
impressions. Diductor impressions elongate tear-shaped, subparallel; adductor 
impressions fairly large and located at the base of the callus filling the deep del- 
thyrial cavity. Between the anterior ends of the diductors occurs a crater with 
steep, coarsely granulose anterior face and moderately deep cavity. Anterior and 
lateral portions of valve strongly granulose. Brachial valve with short but wide 
visceral disk occupying less than the posterior half. 
