58 PALEONTOLOGY OF NEW YORK. 



Distribution. Upper Helderberg group. Corniferous limestone; in the decom- 

 posed chert boulders, Canandaigua, Ontario county. 



Dalmanites (Corycephalus) dentatus. 



PLATE XI a. FIGS. 4-0. 



Dul inniiii.. < to Bakri i r. Amei'. .Tour. Science, vol. xi, 3d Ser., p. 200, plate. 1876. 

 Dalmanitex dentata, Barrett. Amer. J •. Science, vol. xii. 3d Ser., p. 70. 187B. 



Tins interesting Lower Helderberg species is the earliest known form of Dal- 

 manites which presents a development of the ornamentation of the frontal border 

 of the cephalon, a feature which becomes so characteristic of many species of 

 tli is genus in the Upper Helderberg. This ornamentation consists of a single 

 row of acute denticulations, about thirty-five in number, decreasing in size 

 toward the genal extremities, and in their shape indicating the inception of the 

 broader, incisor-like processes of Dalmanites regalis, D. pygmaus and the group 

 represented by D. selenurus. In other respects however Dalmanites dentatus 

 retains the distinctive characters of the Silurian representatives of this genus, 

 that is, forms following the type of Dalmanites Hausmanni, Brongniart, and in 

 respect to features, which are not of an ornamental character, is in harmony 

 w ith its associates in the Lower Helderberg formations, viz., D. nasutus, D. tridens, 

 I), pleuroptyx. The distinct lobation of the glabella, and the absence of any 

 tendency in the first and second lateral lobes to coalesce, the frontal glabellar 

 depression, the long, terete thoracic segments, and the narrow, triangular, cau- 

 date pygidium, are all features, which in their entirety, give a pre-Devonian 

 aspeel to the species. With the opening of the Devonian age some or all of 

 these features appear with modifications so distinct as to require the recogni- 

 tion of sub-divisions in the genus. A notice of Dalmanites dentatus is introduced 

 in this place for purposes of comparison with the Upper Helderberg species, 

 and on account of its frontal ornament is referred with D. regalis and D. pygmeus 

 to the group, Corycephalus. 



Distribution. Lower Helderberg -roup. In the shaly limestone at Port Jer- 

 vis. Orange county; Becraft's Mountain, near Hudson, Columbia county. 



