258 PROCEEDINflS OF THE NATIONAL MUSEUM. vol. xxviii. 



shell. (-ardiiKil oxtreniitios broadly ano-ular, rarely acuminate. Sui- 

 face with radiatiiii;' ribs iitid stria' which may be crossed by concentric 

 j,^rowth lines and stria-. The ribs increase as the shell grows, by 

 interpolation. 



The ventral valve has the umbo more or less elevated over the hinge 

 line, the apex acute and usually incurved. The area is rather broad, 

 tiat or incurved and transversely striated. Teeth short and supported 

 by dental plates that extend to the ])ottom of the valve, bounding 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 iiabelliform 

 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 impunctate, and fibrous as far as known. 



Tyi)e of sahyeims. — OrtJih 'plicatella Hall. 



Ohservatioiis. — The Cambrian species referred to Plectoi'tliis 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. 



The geological range of this subgenus in the Cambrian rocks is from 

 the upper portion of the Middle Cambrian through the Upper Cam- 

 brian, and into the Ordovician, where it terminates in the Hudson 

 River formation. 



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. P. retvojlexa Matthew 

 occurs in strata referred tentatively by Matthew to his Division E2 of 

 the Cape Breton Cambrian section. He first referred the strata to the 

 Ordovician. Of the remaining six species referred to Middle Cam- 

 brian strata /"*. wlc/iitaensis occurs in the upper portion of the Middle 

 Cambrian and base of the Upper Cambrian, and P. indianola also 

 occurs in both Middle and Upper Cambrian. 



From the above statements it will be seen that the first representa- 

 tives of Plectorthis in the Candjrian appear in the Middle Cambrian, 

 and that the greater number of species, 14 out of 21, are Upper Cam- 

 brian forms. 



