BRACHIOPODA. 311 



It may be well to observe that as the species E. subtrigonalis, 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. 



18o4. dionetes? Semenow. Zeitschr. iler deutsch. g-eolog-. Gesellschaft, vol. vi, p. 34.^, pi. v, fig-s. ia-d. 



1862. ChnnHes, Davidson. British Carboniferous Brachiopoda, p. 278, pi. !v, tig-. 13. 



1871. Aulaeorhynchus, Dittmak. Ueberein nenes Brachiopoden-Geschlecht ausdem Berg-kalk ; Verb. 



d. k. Akad. d. Wissensch. St. Petersburg, second ser., vol. vii, p. 1, pi. i. 

 1870. ChonetesJ? Meek and Worthes. Proc. Acad. Nat. Sci. Philadelphia, p. 3u. 

 1873. Isogmmma, Meek and Wokthbn. Geological Survey of Illinois, vol. v, \>. 56S, \,\. xxv, figs. 3a-d. 

 1882. Aalacorhynehiis, Barroi.s. Reeherches sur les Terrains anciens des Asturies et de la Gallice, 



p. 326, pi. xvi, figs. &a-d. 

 1884. Aulacorhynchus, Davidson. British Fossil Brachiopoda ; Appendix to Supplements, ]>. 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 

 nii-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 bracliial 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. Rept. N. Y. State Cab. Nat. Hist., p. 274. 



