296 Lower Helderberg Rocks of Port Jervis, etc. 



layers are crammed with the heads, thoracic segments, and 

 pygidia of trilobites ; the most abundant of which is the 

 new species described by me under the name of Dahnanites 

 dentata (Amer. Journal of Science, Vol. XI, page 200). 

 Homalonotus Vanuxemi and * Dahnanites pleuroptyx are less 

 common, but occur quite frequently. * Chonetes complanata 

 and * Rensselceria mutabilis could not well be thicker than 

 they are in some of these layers. The Chonetes averages 

 about the size of the figure of C. tenuistriata in Prof. Daw- 

 son's Acadian Geology, p. 596 ; and the R. mutabilis is very 

 much larger than any known before, averaging one-half inch 

 from beak to front. The same species from the Dclthyris 

 shale below, is no larger than Prof. Hall's figures of it on 

 plate 45, vol. 3, N. Y. Palaeontology. Besides these, there 

 have been identified *Tentaculites, n. sp., * Loxonema Fitch- 

 iana, Platyceras retrorsum, P. Gebhardii, Holopea sp., 

 *Hyolilhes, n. sp. (described further on), *Pterinea textilis, 

 tSpirifer concinnus, 8. cyclopterus, * Strophomena rhomboid- 

 alts, *S. Conradi, * Strophodonta cavumbona, *Cyrtia ros- 

 trata, Orthis vblata, O. perelegans, O. planoconvexa, *0. 

 subcarinata, Dtscina discus, *D. Conradi, and some other 

 species not yet described. 



Lying immediately below these trilobite layers, is a very 

 hard, cherty layer, full of gasteropoda of the genus Platyce- 

 ras (/)." Favosites conica is found at about the same hori- 

 zon. Loxonema Fitchiana preserves perfectly the fine sig- 

 moidal lines of growth of that species; Platyceras Gebhardii 

 is sometimes as plainly striated as are the living gasteropoda 

 found clinging to the rocks. The Chonetes and the three 

 known species of trilobites likewise preserve their surface 

 markings. 



In my description of Dahnanites dentata, the Delthyris 

 shale was wrongly given for the geological horizon of that 

 species. It should have been the compact layers at the top 

 of the Upper Pentamerus Limestone. I gave it at that time 

 the horizon assigned by Prof. Hall to the most of its associ- 



