576 GEOLOGY OF OHIO. 



mature shell the brachial valve is therefore strongly convex, and the ped- 

 icle valve is decidedly flattened excepting in its posterior half, around 

 the beak. A shallow narrow median depression or sinus extends in the 

 brachial valve from the beak, anteriorly, usually disappearing before 

 reaching the middle of the valve; a similar sinus appears in Orthis 

 fausta. It is a character belonging to the 3 r oung of this species. The 

 marked flattening of the pedicle valve, anteriorly, is seen also in related 

 L,ower Silurian types, where this flattening may even be represented by 

 a more or less developed broad sinus. The numerous, fine, rounded, 

 radiating plications are crossed by fine concentric lines of growth, which 

 in well preserved specimens are seen to be lamellose. These fine lamel- 

 lae are appressed to the anterior portions of the shell, or are but moder- 

 ately elevated above the same, never as much as in Orthis J austa. Finer 

 concentric striae are seen at times between the lamellae. Hinge area of 

 both valves quite flat, that of the pedicle valve a little incurved towards 

 the beak. In the brachial valve it is erect, in the pedicle valve it is in- 

 clined outwards. In both valves the area is marked horizontally by fine 

 close parallel striae of growth, which are crossed at times diagonally by 

 still finer striae, scarcely visible under a lense. Delthyrium open in both 

 valves, the adjoining margin of the hinge area being slightly elevated, 

 forming a minute bordering ridge or striation, best seen in brachial 

 valves, where an anterior continuation of this margin of the hinge area 

 gives rise to the crural plate, which is supported upon a part of the cal- 

 lous thickening which characterizes the hinge region of this valve, inte- 

 riorly. This posterior thickening of the valve occurs in younger speci- 

 imens only near the teeth, as in Orthis fausta, but in older specimens 

 is continued almost to the postero-lateral angles of the shell, strengthen- 

 ing this part of the valve. In the angle between this crural plate and 

 the hinge area is either a deep diagonal notch, or a deep rounded im- 

 pression, forming the socket for the tooth of the opposite valve. The 

 margin of the hinge area bordering the delthyrium in the pedicle valve 

 is also produced anteriorly forming in fact an angular extension of this 

 part of the hinge area ; it is supported by the plate-like elevation of the 

 posterior part of the border defining the muscular area. At times there 

 is the merest trace of a depression in the angle between the tooth and 

 the hinge area. 



The cardinal process of the brachial valve consists as in Orthis fausta 

 of a thin vertical plate posteriorly, considerably thickened at its margin 

 above and in front, this thickening increasing and widening anteriorly, 

 until an ovate process is formed which almost altogether hides its lower 

 more posterior plate-like portion. 



The valves are thin, and frequently show the radiating plications in- 

 teriorly, especially towards the anterior- and lateral margins. The thick- 

 ening of the shell over various areas, to be described later, frequently 

 covers up these plications, especially in the older shells. In the brachial 



