72 PALEONTOLOGY OF OHIO. 



Trematis, Sharpe, they are more nearly related, both in the notched char- 

 acter of the lower valve, and in the marginal beak of the upper valve ; 

 but here their external similarity ends. The nature of the external 

 markings is entirely different from all true species of that genus ; the 

 surface of the shells of this group being simply striated with radiating 

 lines, while those of that genus are characterized by rows or lines of 

 punctures, radiating or eccentric, passing through the outer layers of 

 the shell. The great difference, however, is in the lower valve, and in 

 their habits of growth. The species of Trematis, so far as can be deter- 

 mined from the typical forms, have been bysifferous shells, like those of 

 Discina, attached to foreign bodies by a byssus or plug passing through 

 the foramen of the lower valve, both valves being capable of consider- 

 able motion ; never being permanently attached by their surfaces to for- 

 eign substances, but having a definite form characterizing each species. 

 While these under consideration have been closely adhering bodies, like 

 those of Crania, the lower valve being permanently attached to the sur- 

 face of other shells or foreign bodies, by the whole lower side of the 

 valve becoming fixed and united, and conforming in shape to the sub- 1 

 stance on which it grew. This feature is proved in Orbicula ffilosa, Hall, 

 by numerous examples, most of those from Ohio being attached to valve^ 

 of Strophomena alternata; and in one case no less than five individuals 

 are seen grouped upon a single valve of that shell, all of which sho-v' 

 parts of the lower valve, and all growing firmly to the shell, and con- 

 forming to it in curvature; three of the five retaining portions of th) 

 upper valve. Two other groups are known, of three in each grour, 

 attached to the same species of shell, and retaining parts of the upper 

 valves more or less perfect. We are not aware of a single example from 

 the formation at Cincinnati, Ohio, showing the two valves united, ek- 

 cept in this parasitic condition. In the Utica slates, at Utica, New 

 York, the upper valves of this species (0. ? filoaa) are quite common, bat 

 the lower valve is very seldom seen ; and among collections of scores |0f 

 individuals from these slates, we know of only one group showing the 

 lower valve. This group is clustered around the lower end of an Qr- 

 thoceras, and among about thirty upper valves there are only thirteen 

 lower valves seen, a few of which are not attached to the Orthoceras, but 

 lie quite close to it. 



The muscular markings of Trematis are not clearly understood; all 

 that we know at present being two large, irregular formed imprints near 

 the center of the upper valve. In the form under consideration there 

 are six imprints known in the the upper valve, two of which are large, 

 and situated near the rostral portion of the valve ; two others, minute, 



