CAMBRIAN BRACHIOPODA WALCOTT IO3 



forms show the existence of a transverse apical plate in the del- 

 thyrium of the ventral valve. The name Plecforthis was proposed 

 for a second group, of which Orthis plicatella was made the type, 

 and of this the authors [1892&, p. 194] say: 



"This is a persistent form, which in American faunas, so far as 

 known, is limited to the Trenton and Hudson River lormations. 

 While it retains the strong external ribs of the typical Orthis, these 

 are not invariably simple (0. Ussicosta Hall; O. triplicatella Meek; 

 O. ccquivak'is Hall, not Davidson ; O. jamesi Hall), the cardinal area 

 of the pedicle valve is comparatively low and the valves are sub- 

 equally convex. In the interior, the character of the muscle scars, 

 dental lamellae and cardinal process are essentially the same as in 

 Group I (Orthis), and the minute structure of the shell appears to 

 be in precise agreement with that of O. calligramma, though no evi- 

 dence of tubulose cost?e has been observed. In Orthis jauiesi, which 

 is placed in this association, there is occasionally a deviation toward 

 the resupinate contour exemplified in the Groups IV {Orthis suh- 

 quadrata) and V {Orthis sinuata)." 



In the Cambrian faunas I find a group of species intermediate 

 between the typical forms of Billingsella and of Orthis, as limited 

 by Hall and Clarke [1829&, p. 193], which I have referred for a 

 long time to Hall and Clarke's Plectorthis, placing that genus as a 

 subgenus of Orthis. This Cambrian group of shells, which I now 

 refer to a distinct genus, may be defined as follows, the type of the 

 genus being Orthis rcmnicha Winchell [1886, p. 317] : 



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; sur- 

 face with radiating ribs and striae which may be crossed by concen- 

 tric growth lines and striae. The ribs increase as the shell grows, 

 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 sup- 

 ported by dental plates that extend to the bottom of the valve, bound- 

 ing a space (pseudospondylium) including the main vascular sinuses 

 and area of attachment of the adductor muscle scars. Delthyrium 

 open or partially 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. 



