Report <> 9 eologist. 601 



muscular area is marked by Qabelliform cardinal scais. Inclosing 

 a small adductor impression. 



Iu the brachial valve the cardinal area is narrow, the deltidial 

 covering small and emarginate a1 the center. The cardinal pro- 

 cess is united to the crural plates, the whole forming a vertical 

 suborescentic process, most elevated centrally and notched or 

 divided at its crest, making it bilobed or bidentate, as seen from 

 its inner surface ; on its outer or posterior face each apophysis is 

 deeply grooved, giving the entire cardinal process, from this 

 point of view, a distinctly quadrilobate appearance. The crural 

 plates end more or less abruptly in elevated points, which 

 undoubtedly mark the place of attachment of the crura. The 

 bases of these plates are not produced around the muscular area. 

 Muscular impression nabelliform, very much as in the other 

 valve ; divided medially by a low faint ridge. Shell-substance 

 strongly punctate. 



Type, OrthotheUs crenistria, Phillips (sp.). 



Distribution. Upper Silurian — Carboniferous. 



Hipparionyx, Vanuxem. 1842. 

 (Plate 14, Figs. 17-20.) 



Shell large, subhemispherical. In youth, the pedicle-valve is 

 very slightly convex, but at maturity it is depressed, or concave, 

 over the pallial region. The brachial valve is always very convex. 

 Marginal outline of the valve subcircular. On the pedicle-valve 

 the hinge-line is straight but short, the cardinal area low, and 

 the beak retrorse. The delthyrium is broad, and covered by an 

 imperforate convex deltidium. The teeth are moderately large 

 and are supported by lamellae which extend to the bottom of the 

 umbonal cavity, and are produced into strong ridges entirely 

 surrounding the muscular area. This area is very large and is 

 composed of broad, nabellate diductors inclosing an elongate or 

 cordate adductor impression. There is a low median septum in 

 the umbonal cavity, sepa rating the arms of the cardinal process 

 of the opposite valve, but it is quite short, disappearing at the 

 adductor scar, though sometimes reappearing in front of it. 



In the brachial valve there is no cardinal area. The cardinal 

 process has essentially the same structure as in Orthothetes and 

 Derbya, but is very high, its two branches completely traversing 

 1891. 7tf 153 



