430 Forty-second Report on the State Museum. 



parative paucity in the Niagara, while it is in harmony with their 

 abundance in the Devonian. (Avicula textilis, A. communis, A. obliquata, 

 A. manticula, A. securiformis, A. pauciradiata, etc.). Aviculopecten has 

 a well-developed representation {A. tenuilamellata, A. spinulifera, A. 

 Schoharie, A. umbonata, A. bellula). 



Mytilarcas of large size and gibbous form, belonging to the Devo- 

 nian subgenus Plethomytilus, are abundant in individuals (M. ovata, 

 M. cordiformis). 



Cypricardinia appears in the subdivision of the genus characterized 

 by distant elevated concentric ridges. This group begins in the Niagara 

 (G. undulostriata, C. arataf), is more typically developed in the Lower 

 Helderberg (G. lamellosa), the Schoharie grit (G. planulata), reaches a cul- 

 mination in individual development in the Corniferous and Hamilton 

 species {G. indenta), and is continued to the Chemung (G. arcuata) and 

 even into the Lower Carboniferous {G. consimilis). 



Ilionia (sinuata) is known only here and in the Corniferous limestone. 



Gonocardium has but a meager representation in the single rare 

 species G. inceptum. This genus is but sparingly known in Silurian 

 faunas. 



A large species of Modiomorpha (M. oblonga); undescribed species of 

 of the genera Grammysia* Goniophora* Paracyclas* Lunulicardium* (!) 

 are also known. 



There are no indubitably pree-Devonian types in this element of the 

 fauna. 



Gastropoda. This group is characterized by an immense development 

 of the genus Platyceras, which has been but sparingly represented in the 

 Silurian faunas, but has attained a great abundance in species in the 

 Oriskany, Upper Helderberg and Hamilton. The prevalence of these 

 forms is one of the most striking characters of the Hercynian fauna. 

 The type Orthonychia attains its culmination in the Lower and Upper 

 Helderberg; the nodose species, which become abundant in the Oris- 

 kany and Corniferous, are represented, while the lamellose species and 

 the ventricose, Plat3 r stomoid forms are predominant. The spinose group 

 prevailing in the Corniferous and Hamilton has not been observed 

 here. Noteworthy is the occurrence of a species (P. perlatmn) bear- 

 ing the characters of Kayser's Hercynian genus, Hercynella. 



The genera Strophostylus and Platy stoma have both more abundant 

 representations in earlier and later faunas, and are not diagnostic. 



Loxonema, a post-Silurian type, is of infrequent occurrence. The 

 naticoid genus Holopea is represented by a variety of species. This 



♦These are represented in the collection of Mr. C. E. Beecher and that made by the 

 writer, the latter now the property of Mr. Charles Schuchert. 



