F. R. Coicper Reed — The Genus Trinuckus. 387 



band are not arranged in regular concentric rows, but are placed in 

 front in distinct radial lines with an irregular row of larger pits just 

 inside the girder. Seven or eight pits in a line can be counted in 

 front, but laterally they are indistinct, except along the inner edge, 

 which shows very line closely set shallow short radial grooves, each 

 holding 2-3 very minute pits. The upper surface corresponds with the 

 lower in the arrangement and development of the pits, as I have 

 elsewhere described. 



T. Ziloydi, according to Salter's careful description (op. cit. supra), 

 also appears to have the upper and lower surfaces of the fringe 

 agreeing in their characters. 



In T. concentricus,^ though no strong girder is developed, there are 

 some interesting features in the lower plate. 



Two specimens of the lower plate of this species from the Utica 

 Slate of Cincinnati show the first row of pits lying on the horizontal 

 outer band ; all the other rows lie on the concave inner band. Of 

 these there are 2—3 concentric rows behind the first one in front of 

 the glabella and 4-5 round the cheeks. The pits in the first row 

 are rather smaller and more numerous than those in the second row ; 

 and the pits in all the rows alternate except in the case of the two 

 innermost rows, the fifth row having originated by division of the 

 fourth one and the pits therefore lying radially. On the upper 

 surface of the fringe of this form the two outermost rows tend to be 

 radial nnd also more or less fused together in front but not at the 

 sides. This feature is also seen in a specimen from the Hudson 

 River Group. 



In British specimens attributed to T. concentricus we do not find 

 always complete agreement with the above American examples, and 

 it must be remembered that the species was first described in America 

 by Eaton, so that the transatlantic examples must therefore be 

 regarded as typical. 



The Onny River specimens appear to approach the American some- 

 what closely as regards the fringe, but are not identical. A large 

 series of examples from the Onny River have been examined, and in 

 many cases they liave the shell preserved. The upper and lower 

 plates of the fringe are separated, as described above, and the relations 

 of the pits on the two surfaces can thus be directly observed. On 

 the lower plate the pits in the first row are smaller and usually 

 about one and a half times more numerous than those in the second 

 row, and where this is most marked all the radial arrangement is lost, 

 but there are never any radial sulci. The pits of both the second and 

 third rows are larger than those of the first row ; those of the third 

 row are rather smaller than those of the second row in front, but at 

 the sides become equal. 



The bending up of the inner band is well marked in front behind 

 the second row, but with the decrease in the strength of the girder 

 laterally the differentiation of the inner band dies out, and in 

 section the fringe is here gently biconvex. There is a fourth row 

 of pits alongside the cheeks, splitting into two rows at about the 



1 Eaton, Geol. Text-book, 1834, p. 34, pi. i, fig. 2. 



