CLARK: TRENTON LIMESTONE AT MARTINSBURG. 



13 



hanging stratum of limestone, the anterior margin being embedded 

 in the matrix, and the posterior margin free below the rock. This was 

 probably the position in which the animal lived, besides being the 

 position in which it died, standing erect in the mud, as do the Lingulae 

 to-day. 



Plectorthis sp. 



In the Upper Trenton, near the state road, I found one specimen of 

 Plectorthis, but, although it is in an excellent state of preservation 

 and almost complete, it could not be identified with any described 

 species. In a genus usually so common, I hesitate to make a new 

 species from a single specimen. 



Rafinesquina minnesotensis (Winchell). 



Strophomena minnesotensis Winchell, Ninth ann. rept. Geol. nat. hist, survey 

 Minn., 1881, p. 120. 



This species is rather frequently listed of late, usually from forma- 

 tions of Black River or Stones River age. The horizon of the original 

 specimens was, however. Lower Trenton, and it is gratifying that its 

 first record from New York should be at that horizon. The fossil is 

 very difficult to identify unless one has the interior of the brachial 

 valve. Such a specimen was found, showing the large muscular 

 area, the divergent ridges, and the shallow depression circumscribing 

 the scars. It is also thickly dotted with "ovarian" markings. 



Strophomena trentonensis Winchell & Schuchert. 



Strophomena trentonensis Winchell & Schuchert, Pal. Minn,, 1893, 3, pt. 2, 

 p. 389, pi. 30, fig. 41. 



While this species has a wide distribution, specimens are seldom 

 found. A pedicle valve obtained fifty feet above the base of the 

 Trenton has the muscle scars somewhat smaller than in the Minne- 

 sotan type, and the two divergent ridges which should appear beside 

 the median elevation in the muscular area are lacking. In spite of 

 these variations, the smooth interior of the pedicle valve leaves little 

 doubt of the correctness of the identification. 



