BRACHIOPODA. 
3il 
It may be well to observe that as the species E. suhtrigonalis, upon which the 
genus was established, has a surface quite devoid of the cellular epithelial 
lamina which is so characteristic of all the other known species, the term 
Dictyonella, Hall (1867),* may be found of use in distinguishing the latter 
group of shells. 
Genus AULACORHYNCHUS, Dittmar. 1871. 
PLATE LXXXIII. 
1854. Chonetesf Semenow. Zeitschr. der deutsch. geolog. Gesellschaft, vol. vi, p. 345, pi. v, figs. ia-d. 
1862. Clionetes, Davidson. British Carboniferous Brachiopoda, p. 278, pi. Iv, fig. 13. 
1871. Aulacorhynchus, Dittmar. Ueber ein neues Brachiopoden-Geschlecht aus dem Bergkalk ; Verb. 
d. k. Akad. d. Wissensch. St. Petersburg, second ser., vol. vii, p. 1, pi. i. 
1870. Clionetes?? Meek and Worthen. Proc. Acad. Nat. Sci. Philadelphia, p. 35. 
1873. Isogramma, Meek and Worthen. Geological Survey of Illinois, vol. v, p. 568, i>l. xxv, figs. 3a-d. 
1882. Aulacorhynchus, Barrois. Recherches sur les Terrains anciens des Asturies et de la Gallice, 
p. 326, pi. xvi, figs. 6a-d. 
1884. Aulacorhynchus, Davidson. British Fossil Brachiopoda; Appendix to Supplements, p. 283, 
pi. XX, fig. 22. 
Shells short, transversely elongate or alate; extremities often rounded; 
hinge-line straight, usually making the greatest width of the shell. Valves very 
thin and fragile. Pedicle-valve slightly convex, with traces of a broad, obscure 
m:-dian sinus; brachial valve flat. Surface covered with numerous regular 
and continuous, concentric rounded folds or ridges which are separated by fur¬ 
rows of equal width. 
In the pedicle-valve the character of the articulating processes has not been 
fully ascertained. There appears, however, to have been no cardinal area, and 
but exceedingly small teeth, judging from the analogy of the brachial valve. 
Just within the apex of the valve, which is closely appressed against the oppo¬ 
site one, begins a pair of divergent, elevated ridges, which extend for one-third, 
or even one-half the length of the shell, and enclose a thickened area or plat¬ 
form, which terminates abruptly in a transverse anterior margin. This platform 
is the seat of the adductor and divaricator muscles, and probably rests upon 
the bottom of the valve and is not vaulted. 
In the brachial valve there is a prominent cardinal process from the base of 
which diverge two lateral ridges or socket-walls, lying just within the hinge-line; 
* Twentieth Ann. Kept. N. Y. State Cab. Nat. Hist., p. 274. 
