BRACHIOPODA. 231 



relationship to Clitambonites, as does also the concave plate in the delthyrium 

 of Protorthis Like Protorthis, this genus includes a series of primordial 

 species antedating both Orthis and Clitambonites, but apparently having 

 attained about the same stage in the line of development toward these genera. 

 To the species above mentioned may probably be added the Streptorhynchus ? 

 primordiale, Whitfield,* from the Calciferous beds at Port Cassin, and Orlhisina 

 grandcRva, Billings,f from the Chazy. 



In the lower beds of the Upper Silurian of the Anticosti series occurs the 

 Orthis ? laurentina, Billings, a shell with the interior characters, and the exterior 

 expression of Orthis calligramma, differing only in the presence of deltidia upon 

 both valves, and in this respect resembling Billingsella and Clitambonites. 



Gends protorthis, gen. nov. 



PLATE Vn A, FIGS. 14-21. 



1868. Orthis, Hartt. Dawson's Acadian Geology, Second Edition, p. 644, fig. 233. 

 1884. Orthis, Walcott. Bulletin No. 10, U. S. Geological Survey, p. 17. pi. i, figs. 1, 1 a-d. 

 ? 1884. Orthis. Walcott. Monogi-. U. S. Geological Survey, vol. viii, p. 22, pi. ix, figs. 8, S a. 

 1886. Orthis, Matthew. Transactions Royal Society of Canada, Sec. IV, p. 43, pi. v, figs. 20 a-c ; 



(?) p. 42, pi. v, figs. 18 a-c. 



There is a small group of shells passing under the name of Orthis which 

 should be separated from that genus, and given a new designation, since none 

 of the existing generic groups can properly receive them. Their distinctive 

 characters are as follows : 



Diagnosis. Shells small, transversely subquadrate or semicircular. Hinge- 

 line straight, its length being equal to the greatest width of the valves. Valves 

 unequally biconvex, or sub-planoconvex, the pedicle-valve being the larger. 

 The cardinal area is narrow on both valves, but is higher on the pedicle-valve, 

 and is transected by a broad delthyrium which is closed below by a concave 

 plate apparently produced by the union of the dental lamellae, which are not 

 continued to the bottom of the valve ; teeth distinctly developed. In the 

 brachial valve the cardinal area also bears an open delthyrium ; the dental 



* Bulletin No. 8, American Museum of Natural History, p. 301, pi. xxiv, fig. 7. 1886. 

 t Canadian Naturalist and Geologist, vol. iv, p. 349. 1839. 



