92 NEW YORK STATE MUSEUM 



and clearly also suggest the presence of the mesial fold. Since 

 Sowerby's types were from the-Caradoc of Shropshire, they may be 

 of the same age and identical. For that reason we have designated 

 this particular expression of the species' as t y p u s ; should it be 

 found, however, that the Shropshire and Snake Hill specimens are 

 both local forms, and hence developed independently of each other, 

 those from Snake hill would have to be distinguished by another 

 name, (var. j u g a t a ) . 



Plectambonites centricarinatus no v. 



PI. 4, fig. 7 



The Indian Ladder beds have afforded a rather small type of 

 Plectambonites that is distinguished from the other members of the 

 genus by several peculiar features. The most important of these 

 are the presence of a fold on the pedicel and of a mesial sinus on the 

 brachial valve, the sinus in turn bearing a sharp mesial rib ; another 

 is the prominent cardinal wrinkles. It has the mesial sinus and 

 fold in common with a variety of P. sericeus from Snake 

 hill. The small Indian Ladder type is no longer referable to 

 P. sericeus as a variety, its general form being relatively 

 longer and the valves flatter. This species, in common with P. 

 sericeus var. a s p e r James, from the Rysedorph Hill conglom- 

 erate (title 48, page 18) has the cardinal folds, the tendency to less 

 unequal radiating lines than in the typical P. sericeus and also 

 the acute cardinal angles. Doctor Ulrich has meanwhile pointed 

 out to me that the Rysedorph Hill and James varieties are 

 different. The Indian Ladder form is allied to the Rysedorph Hill 

 variety by the features mentioned and to the P. sericeus 

 from Snake hill in the mesial fold and sinus. 



Plectorthis sp. cf. whitfieldi (N. H. Winchell) 



PI. 4, fig. 8 



A single valve was obtained in the sandy shale of the Snake Hill 

 beds at Snake hill which while agreeing in the surface sculpture 

 with the form here described as Plaesiomys retrorsa, 

 differs greatly in the outline and convexity of the valve. As sug- 



