TRANSACTIONS OF WAGNER 



724 



TERTIARY FAUNA OF FLORIDA 



sonuis being the more inflated. The two valves are usually nearly equal in 

 this respect. 



As there are more ribs in Madisoidiis, they are necessarily narrower, and 

 as the threads are coarser, there are fewer of them on top of the ribs. From 

 this it results that the ribs, as noted by Say, often bear three scabrous threads, 

 sometimes five, young specimens occasionally only two, and similarly in the 

 interspaces, whereas on the backs of the ribs the mesial thread is often more 

 prominent than the others. The three-threaded form was called tricariiiatiis 

 by Conrad, though it appears to have been the original type of Say. The 

 young two-threaded form at first appears very distinct, but such shells ac- 

 quired the third thread with growth. Rarely in this species the threads are 

 fine and uniform, as \x\. Jeffcrsoniiis, but the byssal ear will enable the specimen 

 to be rightly identified. On the whole, it seems as if in Southern specimens 

 the tendency o{ Jeffcrsouiits was to be flatter and have more ribs, and in the 

 Maryland and Virginia form to have fewer ribs and more convex shells. 

 Still, the variety sepioiarius is reported from South Carolina, and a larger 

 series of specimens from Southern localities might show this generalization 

 does not hold uniformly. The young shells of this group would be naturally 

 placed in the section Chlauiys, and the peripheral species in time, such as 

 those of the Eocene and Pliocene, though obviously related to the Miocene 

 type, are perhaps best placed there also. Except in the absence of nodes 

 they are equally close to Nodipcctcii. Even in the Miocene we have species 

 which are strictly intermediate between Placopeclen, Lyropccten, and Chlamys 

 s. s. Hence, no one who carefully studies the various types can feel that a 

 multiplication of genera faithfully represents the facts of nature. 



Pecten (Lyropecten) Madisonius Say. 

 Pecten Madisonius Say, Journ. Acad. Nat. Sci., ist Ser., iv., p. 134, 1S24; Conrad, Fos. 



Medial Tert., p. 48, pi. 24, fig. i, 1840. 

 Pecten tricarinatiis Conrad, Am. Journ. Conch., ill., p. 189, 1867. 

 Pecten fraterniis Conrad, Proc. Acad. Nat. Sci. Phila. for 1862, pp. 2gi, 581, 1863. 



Miocene of New Jersey at Shiloh and Jericho, Cumberland County; 

 of Maryland at St. Mary's River, Greensboro', Choptank River, Langley's 

 Bluff, near Skipton, Barker's Landing, Plum Point, and Calvert Cliffs ; of 

 Virginia at Coggins Point, Temple Place on the York River, Jones's Wharf 

 and Grove Wharf, Suffolk, and Petersburg, and of North Carolina at Snow 

 Hill. 



