432 THE PALEONTOLOGY OF MINNESOTA. 



[Hebertella. 



Trenton group of Canada. 0. (D.) porcata McCoy* has deeper valves, while the area 

 of the ventral valve is more than twice as wide. 0. (D.) retrorsa Salter is a some- 

 what larger species, but differs from 0. (D.) proavita in its retrorse ventral area. 



For the relation of 0. (D.) proavita to other species of the submenus Dinorthis 

 see 0. (D.) subquadrata. 



Formation and locality. Not rare in the upper portion of the Hudson River group at Spring Valley, 

 Minnesota, and Wilmington, Illinois. 



Collectors. W. H. Scofleld, E. O. Ulrich and the writers. 

 Mus. Reg. Nos. 273, 278, 7789. 



Subgenus HEBERTELLA, Hall. 



1892. Hebertella, HALL. Paleontology of New York, vol. viii, pt. i, p. 198. 



Original description: "This division is distinguished both by its external and 

 internal characters; the pedicle valve has a well developed, often much elevated, 

 cardinal area and a long, straight hinge-line; its surface is depressed-convex, always 

 less convex than the opposite valve, which is frequently gibbous or deflected. The 

 surface is covered with a great number of fine, rounded, closely crowded plications 

 which increase rapidly by intercalation, and are crossed by lamellose growth lines 

 and fine concentric striae. On the interior of the pedicle valve the teeth are large 

 and supported by thick lamellae, which are continued as a strong ridge around a 

 short, obcordate muscular area. This area is medially divided by a prominent ridge, 

 upon the summit of which lies the linear scar of the adductors. The flabellate lateral 

 impressions are sometimes divisible into their two components, diductors and 

 adjusters, and in old individuals the impression of the pedicle muscle is often 

 distinct. 



"In the brachial valve the dental sockets are narrow and are enclosed beneath 

 and on the inner side by the strong crural plates. The cardinal process is elongate 

 and simple, sometimes thickened, often crenulate, but not lobed, at its posterior 

 extremity. This process unites with the inner bases of the crural plases and is pro- 

 duced forward as a median ridge dividing the four muscular scars, which are dis- 

 tinctly developed only in old shells. 



"The shell structure is fibrous-impunctate, and the plications of the surface 

 sometimes tubulose." 



Type: Orthis sinunta Hall. 



"Shells of this type of structure are abundant in the Trenton and Hudson faunas 

 and extend upward into the Clinton group, but are not at present known in any 

 later period." 



*S11. Koss. of Ireland, p. 32. pi. in. ftp. 14. 1846; and Pal. Foss.. vol. i. p. !'&. fltf. 111. 1802. 



