NO. 1395. CAMBRIAN BRACHIOPOB A— WALCOTT. 257 



valve is .slightly convex, with h>\v umbo iuul small, incurved apex. The 

 dorsal valve is tiatteiicd in th<' uinbonal region and concave toward the 

 margins; the interior shows a narrow, rounded median furrow, also 

 rounded crural plates that unite with the cardinal process to form a 

 contiiuious ridge covering the anterior portion of the delthjn-ium; the 

 dental sockets appear to have been relatively large and deep: the 

 adductor muscle scars are a little to the front. 



A ventral valve 6 mm. wide has a length of -i mm., and a dorsal 

 valve 5.5 mm. wide is 3 mm. in length. 



Ohse7'vatio))i<. — The concavo-convex valves and the ridge represent- 

 ing the cardinal process place this little shell with the Strophomenoid 

 forms, but as no described genus possesses the characters shown, a 

 subgenus of Sirophoinenn is made to receive it. 



Fortiiation and locality. — Upper Cambrian. Ceratopyge limestone 

 associated with Plectorthix daunu^, Oland, Borgholm, Sweden. 



ORTHIS Dalman 



Subgenus PLECTORTHIS Hall and Clarke. 



Plectorilm Hall and Clarke, Pal. N. Y., VIII, 1892, Pt. 1, p. 194. 



In their subdivision of the genus OrtJiis Dalman, Messrs. Hall and 



Clarke restricted the genus Ortliis to the group of which Orthis cal- 



lactis Dalman is the type, and, among American forms, Ortliis trice- 



nar'ta of the Trenton and Hudson faunas. These forms show the 



existence of a transverse apical plate in the delthyrium of the pedicle 



valve. A second group was placed under the name of Plectorthis 



and called the group of Orthis plicatella^ and of this the authors sav: 



This i.s a persistent form, which in American faunas, so far as known, is limited 

 to the Trenton and Hudson River formations. While it retains the strong external 

 ribs of the typical Orthis, these are not invariably simple ( O. fisslcosla, Hall; 0. irip- 

 llcatelld, Meek; 0. xquivalvis, Hall, not Davidson; 0. JamesI, Hall); the cardinal area 

 of the pedical-valve is comparatively low and the valves are subequally convex. In 

 the interior the character of the muscular scars, dental lamellae and cardinal procesa 

 is essentially the same as in Group 1 (Orthis), and the minute structure of the shell 

 appears to be in precise agreement with that of 0. caUigramma, though no evidence 

 of tubulose costae has been observed. In Orithis Jamesi, which is placed in this 

 association, there is occasionally a deviation toward the resupinate contour exempli- 

 fied in the Oroups IV {Orthis subquadrata) and V {Orthis sinuata). 



In the Cambrian faunas 1 find a group of species intermediate 

 between the typical forms of B/'llmgsella and of Orthis, as limited by 

 Hall and Clarke, which appear to belong to the subgenus Plectorthis. 

 This Cambrian group of shells may be defined as follows, the type of 

 the subgenus being OrtJiis j^licatel la^?i\\., audits Cambrian represent- 

 ative, Orthis 7'emnicha Winchell: 



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 



