FOSSILS OF THE CLINTON GROUP. 573 



tripartite division of the muscular area, similar to that of the valve fig- 

 ured from Hanover, Indiana, described on page 715. The presence 

 of muscular scars of such different character in specimens of the same 

 species seems truly remarkable, but could no doubt be readily explained 

 if the more intimate anatomy of this species were known. This variety 

 reaches a length of 27 mm. and around the margin of the larger individ- 

 uals about 70 radiating plications may be counted. For this variety the 

 name fissi-plicata is suggested. 



At Brown's Quarry a small form 8 or 9 mm. --long is found with sim- 

 ple coarse plications. The ventral valve is strongly convex, and its mus- 

 cular area has a quite prominent border. In one valve the tripartite 

 character of this area is strongly marked, the middle division being a 

 little larger than the two lateral ones. 



Orthis {Herbertelld) fausta. Foerste. 



(Plate 25, Figs. 15 a, b, c, d; 16 a, b ) 



Both valves of medium and almost equal convexity, the greatest cur- 

 vature of the pedicle valve being about two-fifths the distance from the 

 beak. The brachial valve has a small, narrow, ill-defined median depres- 

 sion or sinus posteriorly which disappears usually towards the middle of 

 the valve and is, therefore, a character retained only in the young of this 

 species. The adult forms of the ancestral types of the species, of 

 Lower Silurian Age, had usually a distinct, though often shallow sinus, 

 a character perhaps retained in part only by the young of the present 

 species. Both valves marked by rather sharp radiating plications/crossed 

 by lines of growth, which in some specimens are merely fine striae, but 

 -vhich in others are more prominent, developing into lamellose striations, 

 the zigzaging lamellae being either more or less appressed to the shell, or 

 distinctly elevated above the same, in the latter case giving the shell a 

 highly ornate appearance. Hinge area of the brachial valve low and 

 erect, that of the pedicle valve much higher, outwardly inclined below, 

 incurved above, rarely marked by horizontal, parallel striae of growth. 

 Delthyrium open in both valves, in the brachial valve bordered by the 

 raised margin of the hinge area, which here forms a sort of ridge increas- 

 ing in size anteriorly, jutting beyond the outline of the hinge area and 

 forming the crural plate. A diagonal notch in the angle between the 

 tooth and the hinge area receives the tooth of the other valve. The 

 sharp, inner margin of the hinge area of the pedicle valve where it bor- 

 ders upon the delthyrium is also prolonged anteriorly, forming a plate- 

 like tooth, resting upon the plate-like walls bounding the muscular area 

 laterally, as a support; no notch separates this tooth from the hinge area, 

 of which it is practically an outgrowth. 



The cardinal process of the brachial valve is a very thin erect plate 

 along its posterior line of attachment, the curved upper margin of this 



