PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER ~ 807 
Measurements in mm— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
Piolotype ets fet. 3 2.9 2.6 5.0 6.4 0.4 0.8 
Paratype (i110925a).... 3.8 ? 6.4 8.9 ? ? 0.7 
4 (110925b).... 3.9 ? 6.7 7.8 ? 1.1 
Types.—Holotype: 117475b; figured paratypes: 110g25a-d, 117475a; un- 
figured paratype: 117475c. 
Horizon and locality—Pratt Ferry formation in Alabama: 0.2 mile southeast 
of Pratt Ferry, Blocton (15’) Quadrangle. 
Discussion.—This species is characterized by its low convexity and the pres- 
ence of 3 prominent costellae, 1 median one and 2 lateral ones. These features 
distinguish this species from A. inaequistriata which is more convex and has 
numerous large costellae. 
Genus EOPLECTODONTA Kozlowski, 1929 
Eoplectodonta KozLowsk1, Palaeont. Polonica, vol. 1, No. 1, pp. 30, 113, 1920. 
This genus was proposed for sowerbyelloids having a partially denticulate 
hinge line. Actually the genus has some combinations of ornamentation and in- 
terior characters that will serve to distinguish it from Sowerbyella and other 
members of the Plectambonitacea. 
All the known species of Eoplectodonta are characterized by ornamentation 
in which larger costellae define bands of smaller radii of greater or less width. 
The species also are characterized by a prominent oblique wrinkling of the pos- 
terior margins. These features occur in some other genera, such as Sowerbyella, 
but in that genus the wrinkling of the hinge is usually a variable feature whereas 
it appears to be constant in Eoplectodonta. Furthermore, no species of Eoplecto- 
donta is now known in which the finer costellae are beaded as in American 
Porterfield, Wilderness, and Trenton species, 
Inside the pedicle valve the arrangement of the muscle and pallial trunks is 
like that of Sowerbyella, but Eoplectodonta does not have the forked arrange- 
ment at the anterior end of the septum so common in the former genus. Inside 
the brachial valve the feature of most importance is the development of the 
median septum to a stage equal to or surpassing that of the lateral septa. This 
produces 3 almost uniform septa which are only slightly divergent. These rise 
to a crest near the center of the valve as in Sowerbyella and usually have a steep 
papillose anterior slope. The triradiate pattern of the septa is very distinctive. 
This feature appears in one species of Sowerbyella and suggests that Sowerbyella 
is the ancestor of Eoplectodonta. 
Eoplectodonta is well represented in the British Llandovery fauna which in- 
cludes E. praecursor, the genotype, and E. duplicata. Plectambonites schmidti 
Tgrnquist of Etage 5b from Norway appears to belong here. The American 
species are much older than the British forms. 
