MARCELLUS LIMESTONES OF LANCASTER 165 



spirifer Sowerby 

 Spirifer mucronatus (Conrad) 



Pal. N. Y. 4:216, pi. 34 



Fragments of these shells are found in beds V and VI at 

 Plumibottom creek, and large and fairly well preserved speci- 

 mens in the same beds at Cayuga creek. 



Spirifer (Martinia) subumbona Hall 



Pal. N. Y. 4:234, pi. 33 



Young and exfoliated shells of this species are with difficulty 

 distinguished from Ambocoelia nana, but the speci- 

 mens are proportionally broader and the sinus less pronounced. 

 The adult is larger, an average sized shell having the dimensions : 

 length 9 mm, width 11 mm. These shells are extremely common 

 in bed II, 



RETICULARIA McCoy 



Reticularia fimbriata (Conrad) 



Pal. K Y. 4:214, pi. 33 



This species is represented by very perfect molds of the inter- 

 ior in which a part of the shell is sometimes retained. A speci- 

 men of average size measures: length 20 mm, width 29 mm. The 

 shells are fairly common in beds VI and VII. 



AMBOCOELIA Hall 



Ambocoelia nana Grabau 



PI. 9, fig. 21-23 



Geology and paleontology of Eighteen-mile creek, p. 217 

 Perfect specimens agreeing with the description of the type, 

 abound in bed I, but in the shales below, specimens occur which 

 show two types of variation. Here the more common form has 

 the usual proportions of Ambocoelia nana, but the 

 pedicle valve has an exceptionally deep sinus with strong and 

 closely set spines. The spine pits are deep and not greatly elon- 

 gated. The brachial valve has the typical convexity and broad 

 shallow sinus. Average dimensions are: length 6 mm, width 



