BRACmOPODA. 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. 

 VI. Group of Orthis strophomenoides, Hall. 



PLATE Va, figs. 22-27. 



1S57. Orthis, Hall. Tenth Rept. N. Y. State Cab. Nat. Hist., p. 46. 



1859. 0)-ifo's,'HALL. Pateontology of N. Y., vol. iii, pp. 177, 4S1, pi. xiv, fig. 2 and pi. xxiii, fig-. 7. 



1869. Orthis, Saffokd. Geology of Tennessee, pp. 328, 533. 



1883. OrthostropMa, Hall. Rept. N. Y. State Geol. for 1882, 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- 



