568 GEOLOGY OF OHIO. 



pedicle valve. The valves alw^s adhere by one face or the other to trie 

 matrix; the hinge area is not shown, the shell is usually more or less ex- 

 foliated, the muscular area can not be detected and so it becomes impossible 

 at times to tell whether the exterior or the interior of a valve is pres- 

 ent, even when both are known to be present, among this partly ex- 

 foliated material. On the same piece of rock with the cabinet speci- 

 men above mentioned, and in one collected by the writer at Todd's Fork, 

 Ohio, (Plate 27, Fig. 34) flattish valves or rather moderately convex 

 valves are found presenting an unequivocal exterior surface with numerous 

 fine radiating plications or striae, and much finer concentric striae of 

 growth visible only in good specimens and with a lense. The radiating 

 striae are quite equal in size except for the intercalated less prominent 

 striae, which do not however occur in groups between the larger speci- 

 mens, but are found usually only one or two at a time between the larger 

 striae. Judging from the similarity of their surface ornamentation, and 

 their similar size, the specimens should belong to the same species, while 

 their being so much less convex makes their identification as pedicle 

 valves exceedingly probable. We have here therefore pedicle valves 

 which are not concave but moderate!)- convex. As a rule the 

 brachial valve shows a very slight median depression near the beak, 

 while the pedicle valve does not in the same region in the Todd's 

 Fork specimen, which is regarded typical. In the cabinet speci- 

 men in the Ohio State University there seems to be a very shallow 

 median depression. 



This species occurs at the Soldiers' Home and Todd's Fork in 

 Ohio, and more abundantly at Hanover, Indiana. The double convex- 

 ity of this shell, if determined with greater certainty from a greater supply 

 of material, will remove it from the more typical strophemenoid shells, 

 and will place it apparently with Orthothetes. 



Strophomena (Orthothetes) tenuis, Hall. 



(Plate 27; Figs. 31, 32, 38.) 



Shells of this species are quite readily recognized by their flattish 

 pedicle valves, moderately convex in the immediate neighborhood of the 

 beak which is therefore a little elevated. The brachial valve on the con- 

 trary always shows pronounced convexity, although this convexity never 

 reaches the point of the shell being even moderate gibbous. By digging 

 away the shell from the matrix the pedicle valve was found to have its 

 muscular area sharply outlined postero-laterally by a ridge on either side, 

 which began near the beak where the teeth usually are, and continued 

 antero-laterally, soon disappearing. The postero-lateral outlines form an 

 angle of fifty or fifty-five degrees near the beak. A low rather broad 

 median elevation is seen in the posterior part of this area but soon dis- 

 appears anteriorly. Antero-laterally and anteriorly the muscular area 

 does not show any boundaries in our specimens. The brachial valve 



