PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 627 
DREPANORHYNCHA Cooper, new genus 
(Greek drepanon, sickle; rhynchus, beak) 
Shell rhynchonelloid in outline and profile; hinge narrow, both valves pro- 
vided with narrow interareas ; delthyrium open; anterior commissure uniplicate ; 
surface costate; shell fibrous, impunctate. 
Pedicle valve with deep delthyrial cavity bounded by slightly convergent dental 
plates; teeth large; musculature indistinct. Brachial valve with deep sockets, 
divided hinge plate with concave outer hinge plates, long crural bases, and long, 
slender, curved crura. Crural bases united with posterior of notothyrial cavity 
and lying obliquely under the notothyrial edge. Median septum lacking. 
Genotype——Porambonites ottawaensis Billings=Drepanorhyncha ottawaensis 
(Billings), Paleozoic fossils, vol. 1, p. 140, fig. 117 (adv. sheets), 1862. 
Discussion—This species was at first mistakenly placed under the genus 
Porambonites with which it has nothing in common externally or in the interior. 
Later it was transferred to Camerella, but it is no more at home in that niche 
than it is in the first category. Later Schuchert and Cooper, 1932 (plate 16) 
transferred P. ottawaensis to Orthorhynchula because it possessed a pedicle inter- 
area and a divided hinge plate with concave crural supports. Reexamination 
of the types of P. ottawaensis and additional material, together with study of 
better interiors of Orthorhynchula, show that the two types have little in common 
besides the interareas. 
Drepanorhyncha is characterized by the presence in the pedicle valve of 
elongate dental plates spaced fairly closely together making a deep and narrow 
delthyrial chamber. The muscles are separated by a low and narrow median 
ridge. In the brachial valve the plates bearing the crura are concave and are 
supported by attachment to the posterior of the notothyrial cavity and the edge 
of the notothyrial margin facing the brachial valve. The crura are exceptionally 
long and slender. No median septum nor cardinal process is present. 
In contrast to Drepanorhyncha the dental plates of Orthorhynchula are short 
and receding and are attached closely to the lateral walls of the valve. In the 
brachial valve the hinge plate is divided into two concave outer hinge plates 
which are concave and resemble those of Drepanorhyncha although they lie in 
the valve differently, presenting the concave face toward the pedicle valve, 
whereas in Drepanorhyncha the same plates lie obliquely facing each other. In 
Orthorhynchula, furthermore, the hinge plates are attached to a median swelling 
that bears a typical orthoid cardinal process situated between the two plates. The 
cardinal process has a stout shaft and a compressed and crenulated myophore 
like that of Hebertella. The median ridge, although low, is present and prominent. 
As far as known the genus Drepanorhyncha occurs only in Trenton rocks in 
New York and Ontario. 
DREPANORHYNCHA AMBIGUA (Hall) 
Plate 128, G, figures 37-39 
Airypa ambigua Hatt, Pal. New York, vol. 1, p. 143, pl. 33, figs. 8, 9, 1847—EmMons, 
Amer. geology, vol. 1, pt. 2, p. 190, pl. 10, figs. 8, 8c, 9, 1855. 
