562 



HISTORICAL GEOLOGY. 



Characteristic Species. 



Some of the species of the subdivisions fi-om eastern New York are : — 



1. Lower Pentamerus. — Lepadocrinus Gebhardi, Bhynehonella semiplicata, Penta- 

 merus galeatus, Meristella Icevis (also in the Shaly), Favosites Heklerherf/ke. 



2. Shaly Limestone. — Hemipronites radiatus^ H. punctuliferus, Orthis oblata, 0. 

 planoconvexa, 0. varica, Eatonia singularis, Spirifer macropleurus, iSp. perlamellosiis, 

 Sp. cyclopterus, Platyceras ventricosum, Dalmanites pleuropteryx. 



3. Upper Pentamerus. — Pentamerus pseudogaleatus, Bhynehonella ventricosa, 

 B. nobilis H. 



820 -■ .o.. 820-825. 



Acidaspis tuberculata, Fig. 820, upper view of head of an adult (IJ) ; 821, segment of thorax (1|) ; 822, pygi- 

 dium of adult (x §) ; 823, pygldium of young (x 3J) ; 824, dorsal view of larva (x 30) ; 825, profile view of 

 same (x 30). Beecher. 



Atrypa recticularis and Leptcena rhomboidalis of the Niagara are supposed to be the 

 only species that are continued into the Lower Helderberg. 

 Some of the genera are as follows : — 



1. Spongiozoans. — Stromatopora^ very common in the Lower Pentamerus, con- 

 stituting in some places a bed 3 or 4 feet thick. Also Ischadites, Hindia, Beceptaculites 

 infundibuliformis. 



2. Actinozoans. — Strep>telasma, Zaphrentis, Michelinia (begins), Favosites (of which 

 F. Helderbergue is often a foot across), Aulopora. 



3. Echinoderms. — Lepadocrinus, Anomalocystites, Sphcerocystites, Melocrinus, Cor- 

 dylocrinus, Edriocrinus, Homocrinus, Coronocrinus, Protaster (P. Forbesii H.). 



4. Brachiopods. — Orthis (many species), Strophomena (Leptcena of Hall, 1892), 

 Stropheodonta, Chonetes (2), Bhynehonella (numerous species), Pentamerus, Eatonia, 

 Anastrophia, Spirifer (veiy many, some with dichotomizing ribs, a feature especially of 

 later time), Cyrtina, Trematospira (several species) , Meristella, Atrypa, Bensselceria. 



5. Mollusks. — Lamellibranchs. — Avicidopeeten, Mytilarca, Megambonia, Cypricar- 

 dinia, Gonocardium. 



Gastropods. — Platyeeras, Platystoma, Holopea, Loxonema, Murchisonia, Pleuroto- 

 maria, Strophostyhis, Euomphalus, Bucania. 



Cephalopods. — Orthoceras of several species, Oncoceras ovoides H., Cyrtoceras. 

 Pteropods. — Conulana, Tentaculites, and Hyolithes. 



6. Crustaceans. — Besides those mentioned, Bronteus, Homalonotus, Cyphaspis, 

 Proetus. The Ostracoid genera, Leperditia, Beyrichia, uEchmina. 



The Lower Helderberg beds, near Eastport, southeastern Maine, include Favosites 

 cervicornis, Leptcena rhomboidalis, Chonetes Novascoticus, Bhynehonella Wilsoni, Atrypa 

 reticAdaris, Orbicluoidea tenuilamellata, Spirifer sulcatus, Orthis eleganttda, Avinila 

 naviformis, Calymene Blumenba.chii, Homalonotus Dawsoni, Conulites flexuosus, Tenia- 

 culites distans, Beyrichia lata, and others (W. A. Rogers). 



