35 Ithaca Group 35 



Cyrtbia hamiltonensi^ Hall. 



Very abundant in the upper part of the Ithaca group. 



The large number of specimens of this species which have 

 been examined show but slight tendency to var}^ except in size. 

 Average specimens have a width of about ^ inch along the hinge 

 line; the longest noticed measured seven-tenths of an inch. 



Ambocoelia iimbo7iata Conrad. 

 Abundant in the Ithaca group. 



Strophodonta miicronata Hall. 



Very abundant in the upper Ithaca group. It also occurs 

 through most of the Portage. 



Strophodonta perplana var. nervosa Hall. 



Occurs in the Ithaca group but is less common than the pre- 

 ceding. 



Strophodo7ita demissaf Con. 



A rare species in the Ithaca group. 



Produdus {Produflella) hallamis Walcott. 



Two or three specimens of this species have been found in 

 the upper part of the Ithaca group. 



ProdiiHella speciosa Hall. 



This is an abundant and charadleristic species of the Ithaca 

 group. The larger specimens measure from four-fifths to one 

 inch in width. The most abundant species associated with it in 

 the Ithaca group are Strophodonta inucronata, Spirifer niesacos- 

 talis, and Cyrtina haniiltonensis. 



ProduFtella truncata Hall. 



This is a common species in the Ithaca shale. It seems to 

 differ from P. speciosa only in size, some specimens measuring 

 not more than one-tenth of an inch, while those of average size 

 are from three-tenths to two-fifths of an inch in width. In the 

 lower part of the Ithaca group, forms occur which seem to be 

 intermediate between P. truncata and P. speciosa. 



Spirifer mesacostalis Hall. 



Abundant in the Ithaca group. 5. mesacostalis shows a large 

 amount of variation in specimens from the same horizon. Vari- 

 ation occurs principally in connexion with four different charac- 

 ters, — the number of plications, the extent of the hinge line, 



