104 SMITHSONIAN MISCELIvANKOUS COLLECTIONS VOL. 53 



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 ground-mass. 

 Sections vertical to the outer surface 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 

 fibers, and a dense granular ground-mass 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 Ortliis (Dal- 

 nianella) parva and allied punctate orthoids ; the openings are usu- 

 ally indicated by minute scattered dark spots. 



Type of Genus. — Orthis remnicha Winchell [1886, p. 317]. 



Observations. — The Cambrian species referred to Boorthis 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. Boorthis may be distinguished from Orthis (s. s.) 

 by (i) its ribs increasing by interpolation; (2) its strongly defined 

 pseudospondylium; (3) relatively thin shell; and (4) its dense, non- 

 fibrous shell structure. The last three characters also distinguish 

 it from Plectorthis and other subgenera of Orthis. Boorthis may 

 be considered as the possible connecting line between Billingsclla and 

 the orthoids of the Ordovician. 



The geological range of Boorthis 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 Paradoxidcs in Bohemia, and it is not improbable 

 that they will be found to belong to some other genus. The remain- 

 ing species referred to the Middle Cambrian are B. z^'ichitensis (Wal- 

 cott) [1905a, p. 271], which occurs in the upper portion of the Mid- 

 dle Cambrian and base of the Upper Cambrian, and B. hastingscnsis 

 (Walcott) [1905a, p. 263], which occurs in the Middle Cambrian 

 (Paradoxidcs zone). 



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

 tives of Boor til is in the Cambrian appear in the upper portion of 

 the Middle Cambrian, and that the greater number of species, 21 out 

 of 31, are Upper Cambrian forms. 



