BILLINGSELLID^. 773 



This Cambrian group of shells which I now refer to a distinct genus may be defined as follows, 

 the type of the genus being "OrtTiis remnicha" Winchell (Pis. XCI and XCII): 



Diagnosis. — Shells subquadrate to transversely elongate; with or without median fold 

 and sinus; valves subequally convex. Hinge line straight, usually forming nearly the greatest 

 diameter of the shell. Cardinal extremities broadly angular, rarely acuminate. Surface with 

 radiating ribs and stride which may be crossed hj concentric growth lines and striae. The ribs 

 increase by interpolation. 



The ventral valve has the umbo more or less elevated over a hinge line, the apex acute 

 and usually incurved. The area is rather broad, flat or incurved, and transversely striated. 

 Teeth short and supported by dental plates that extend to the bottom of the valve, bounding 

 a space (pseudospondylium) including the main vascular sinuses and area of attachment of 

 the adductor muscle scars. Delthyrium open or partly closed by a convex deltidium. The 

 adductor muscle scars are included within a narrow median area beneath the umbo on each 

 side of the median line, and the diductors in a more or less flabelliform area outside of the 

 main vascular sinuses. Pedicle scars unknown. 



Dorsal valve with low umbo and slightly incurved apex; area well developed, with a 

 broad delthyrium. Deltidial cavity with a straight, simple, cardinal process. Dental sockets 

 small, with short crura. The adductor muscle scars are small, the anterior being nearer the 

 median ridge, which usually extends forward from the base of the cardinal process. 



Shell structure dense, with a minutely granular groundmass. Sections vertical to the 

 outer surface, however, show a few laminations of growth, but no fine fibers; sections on the 

 plane of the surface show a few coarse irregular fibers resembling matted wood pulp ; a dense 

 granular groundmass that is penetrated here and there by irregular openings of varying size. 

 The openings or pores appear to be confined to one or more lamellae of the shell and not to 

 pass through it from inner to outer surface, as in Orthis (Dalmanella) parva and allied punctate 

 orthoids. The openings are usually indicated by minute scattered dark spots. 



Type. — Orthis remnicha Winchell. 



Observations. — The Cambrian species referred to Eoorthis have relatively thin shells that 

 retain on the interior surfaces but slight traces of the muscle scars and vascular markings, 

 except in the umbonal cavity. Eoorthis may be distinguished from Orthis (s. s.) by (1) its ribs 

 increasing by interpolation; (2) its strongly defined pseudospondylium; (3) its relatively thin 

 shell; and (4) its dense, nonfibrous shell structure. The last three characters also distinguish 

 it from Plectorthis and other subgenera of Orthis. Eoorthis may be considered as the possible 

 connecting link between Billingsella and the orthoids of the Ordovician. 



The geological range of Eoorthis is from the upper portion of the Middle Cambrian through 

 the Upper Cambrian and into the lower portion of the Ordovician. 



Two of the species from strata referred to the Middle Cambrian are represented by material 

 too imperfect for specific description; they occur with Paradoxides in Bohemia, and it is not 

 improbable that they will be found to belong to some other genus. The remaining one of 

 the three species referred to the Middle Cambrian is E. wichitaensis, which occurs in the upper 

 portion of the Middle Cambrian and base of the Upper Cambrian, and E. hastingsensis, which 

 occurs in the Middle Cambrian (Paradoxides zone). 



Eoorthis agreste (Walcott). 



Plate LXXXIX, figures 6, 6a. 



Orthis (Plectorthis) agreste Walcott, 1906, Proc. U. S. Nat. Mus., vol. 30, p. 570. (Described as below as a new species.) 



Shell transverse, subsemicircular ; a ventral valve 9 mm. in length has a width of 12 mm. 

 and a hinge line 10.5 mm. in length; ventral valve moderately and regularly convex, with the 

 apex curved down to an area that is slightly inclined forward from the hinge line. The details 

 of the cardinal area are unknown. 



