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 tw^ice 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 subgenus Dinorthis 
see 0. (D.) suhquadrata. 
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. Pal^ontologj' of New York, vol. viii, pt. 1, 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 strite. 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 
. adjustors, 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 sinuota 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." 
* Sil. Foss. of Ireland . p. 32. pi. iii, fig. 14. and Pal. Foss., vol. i. p. V.&. tig. Ill, IStB. 
