BRACHIOPODA. 
199 
the crural plates and is produced forward as a median ridge dividing the four 
muscular scars, which are distinctly developed only in old shells. 
The shell-structure is fibrous-impunctate, and the plications of the surface 
sometimes tubulose. 
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. 
ORTHOSTROPHIA, Hall. 1883. 
YI. Group of Orthis strophomenoides, Hall. 
PLATE VA, PIGS. 22-27. 
1S57. Orthis, Hall. Tenth Rept. N. Y. State Cab. Nat. Hist., p. 46. 
1859. Orthis, Hall. Palaeontology of N. Y., vol. iii, pp. 177, 481, pi. xiv, fig. 2 and pi. xxiii, fig. 7. 
1869. Orthis, Safford. Geology of Tennessee, pp. 328, 533. 
1883. Orthostrophia, Hall. Rept. N. Y. State Geol. for 1S82, pi. xxxvi, figs. 3 2-34. 
This section is represented by impunctate, resupinate shells, having the 
external characters of those included in Groups IV and V. In the interior 
of the pedicle-valve the muscular area is deeply excavated, very confined, and 
limited almost to the narrow space between the dental lamellae. The adductor 
scars appear to be extremely minute and linear, while the cardinals occupy 
the rest of the area. The margin of this area is thickened and elevated, and 
is so contracted as to convey, at first, the impression that it is no more than a 
rather large rostral cavity. In the opposite valve the muscular area is sim¬ 
ilarly confined but the quadrilobate character of the impression is very decided. 
The lateral components are divided by a strong median ridge, and the entire 
area is deeply impressed in the substance of the shell. The cardinal process is 
elongate and simple at its posterior extremity, the crural plates and crura strong. 
The vascular and ovarian markings are a conspicuous feature of these shells 
and are almost invariably developed with remarkable distinctness. In the 
pedicle-valve two or three large vascular trunks originate near the anterior 
edge of the muscular area, divide a few times in their passage over the pallial 
region, the branches rapidly multiplying near the margin. In the opposite 
valve the main sinuses are four in number, originating in pairs at the ante- 
