LOWER SILURIC SHALES OF THE MOHAWK VALLEY 93 



gested to me by Doctor Ulrich, it may possibly be the ventral valve of 

 a Plectorthis, like P. whitfieldi (N. H. Winchell). The 

 costae are remarkably sharp, separated by intervals of equal width; 

 they increase twice by interplantation and number forty-six along 

 the margin. Concentric striae are seen in the intervals. The orig- 

 inal is an impression on a slab and the marginal parts may not be 

 entirely preserved and hence the outline not as figured. 



Plaesiomys retrorsa (Salter) 

 PL 4, fig. 9-12 



The Snake Hill beds contain, at Snake hill and in other locali- 

 ties large orthid shells, which however in the shales are always 

 badly crushed and flattened. The gritty sandstone and conglomerate 

 beds at Snake hill have afforded some excellently preserved speci- 

 mens which are here reproduced. These latter show the character- 

 istic features of Salter's P. retrorsa in outline and especially 

 in the retrorsion of the umbo of the pedicel valve and the forward 

 inclination of the cardinal area of the pedicel valve which lat- 

 ter feature distinguishes this species from the closely related 

 P. porcata (see title 5, page 136). The surface sculpture 

 corresponds to that of P. retrorsa as described by Billings 

 (loc. cit), Meek (title 6, page 93) and Cumings (title 57, page 

 902). It consists of sharp ribs (about fifty or more along the 

 margin) that increase by intercalation on the brachial valve but 

 are nearly straight on all parts and increase by bifurcation on the 

 pedicel valve. Closely arranged fine concentric lines are seen in 

 the interspaces while the ribs are smooth or but obscurely marked. 



The hinge of the brachial valve which hitherto has not been 

 observed, is reproduced in figure 10. It is not essentially different 

 from that of P. loricata figured by Hall and Clarke (title 18, 

 plate 5 A, figure 33) and very remarkably like that of 

 Hebertella sinuata figured ibidem. A larger valve 

 (plate 4, figure 13) has the characters rather of P. porcata 

 McCoy (see title 5, page 135) than of the closely related re- 

 trorsa. Both P. retrorsa and P. porcata occur also 

 in the Trenton limestone at Ottawa ; the appearance of these 

 European species in the Snake Hill beds is therefore not a strange 

 feature. 



