LOWER SILURIC SHALES OF THE MOHAWK VALLEY QI 



Sagenella ambigua Walcott. According to informa- 

 tion from Doctor Ulrich, these growths represent one or more 

 species of Spatiopora, Sagenella ambigua being one 

 of them. A different incrustation on a cephalopod from the Snake 

 Hill beds is above described asPaleschara ulrichi. 



Lingula rectilateralis Emmons 



PI. 4, fig. i 



This species which was originally identified by Hall with 

 L. quadrata Eichwald occurs in good specimens in the Sche- 

 nectady beds at the Dettbarn quarry at Schenectady and at Rexford 

 Flats. Hall cited the species only from the middle and upper 

 Trenton, but according to Schuchert it ranges from the Trenton into 

 the Lorraine. 



Orbiculoidea tenuistriata (Ulrich) 

 PI. 4, fig- 2 



A single specimen of this species was found at the top of the 

 Frankfort shale in the Frankfort gulf. It has been identified by 

 Doctor Ulrich with his species described in 1878 (Jour. Cin. Soc. 

 Nat. Hist. I, p. 96, pi. 4, fig. 10) from beds of Covington, Ky., 

 that are now referred to the middle part of the Eden group of the 

 Cincinnati region. 



Plectambonites sericeus (Sowerby) typus 

 PI. 4, fig. 3-6 



The Snake Hill shale at Snake hill contains a possible but local 

 variety of PI. sericeus which markedly differs from the 

 currently accepted expression of this extremely variable species in 

 possessing a well-defined mesial fold on the pedicel valve and a 

 corresponding sinus on the brachial valve, as shown in the specimens 

 here reproduced. In other features, notably the general outline and 

 surface sculpture, the specimens do not appear to differ from the 

 general aspect of the species. While we have found this feature 

 neither figured nor described of any of the American representatives 

 of the species, Sowerby's original figures in " Siluric System " 

 exhibit the same outline and relative size as our Snake Hill variety 



