266 I'ROCEEDINaS OF THE NATIONAL MUSEUM. vol. xxviii. 



Doctor Matthew very kindly sent me the types of this species. 

 There is little that can he added to his very complete description. As 

 far as known, the characters of the shell are those of the group of 

 species referred to, Plectorthix. 



PLECTORTHIS KAYSERI, new species. 



This is a larger shell than PlectortluH Ihiiiwrsmiu^ with which it is 

 associated. Its surface is marked by numerous line radiating stria?, 4 

 to 5 in a distan(;e of 1 mm., and the inner layers of the shell appear to 

 be minutely punctate. The ventral valve is more convex than the 

 dorsal, the latter being nearly flat in young- shells. Nothing is known 

 of the interior except one cast of the pseudospondylium of the dorsal 

 valve, which is larger in proportion than that of P. Unnarmoni. 



The largest ventral valve has a length of 14 mm.; width, 20 mm. 



This species belongs in the group of Plectorthis represented by 

 P. desniojjleura and P. UnnarssonL 



The species is named after Dr. Emanuel Kayser. 



Formation and locality. — Upper Cambrian. Lower part of Chao 

 Mi Tien (?) limestone, 2.7 miles southwest of Yen Chuang, and 

 Chao Mi Tien, Province of Shantung, China. Collected by Eliot 

 Black welder and Bailey Willis of the Carnegie Institution Expedi- 

 tion, 1903. 



PLECTORTHIS LINNARSSONI Kayser. 



Orthls Ihvnarxsoiii Kayser, Cambrische Brach., von Liau-Tung, China, Rich- 

 thofen, IV, 1883, p. 34, pi. iii, fig. 1. 



Original description. — Shell seniielliptic in outline, broader than long, with hinge 

 edge straight, corresponding to the greatest breadth of the shell. Large [ventral] 

 valve moderately convex. Small [dorsal] valve slightly arched, with a sinus devel- 

 oping already at the umbo, and growing quite broad and deep toward the edge. 

 Umbo of ventral valve small, area very low. Surface of shell covered with rather 

 sharp ribs, very variable in strength, separated by narrow furrows. By reason of 

 repeated splitting, beginning close to the umbo, the ribs appear rather as bundles of 

 ribs. On the matrix these bundles appear as broad, obtuse-angled folds, which, 

 owing to repeated marginal splitting in even a higher degree than on the shell 

 itself, appear as bundles of ribs of very unlike strength. When the surface is well 

 preserved, a delicate concentric growth striation is perceptible. 



Doctor Kayser compares this shell with Orthh hicksi Salter, and 

 (). exporecta Linnar.^son, on account of the surface characters. It does 

 not appear to be very closely related to either species, but it is in 

 many respects allied to Plectorthis desmople^ira2indL P. wicldtaensls. P. 

 Unnarssoni differs from both in being more transverse. 



Formation and locaUty. — Middle Cambrian. Lower part of Chao Mi 

 Tien (?) limestone, 2.7 miles southwest of Yen Chuang, Province of 

 Shantung, China. Collected by Eliot Black welder and Bailey Willis, 

 of the Carnegie Institution Expedition, 1903. 



