NIAGARA GROUP. 



309 



503. 1. HOMALONOTUS DELPHINOCEPHALUS. 



PL. LXVIII. Figs. 1 - 14. 



Trimerus delphinocephalus, Green, Monograph, 1832, pag. 82, pi. 1, tig. 1 ; Idem, Monthly Am. 

 Journal of Geology, p. 560. 



— — Haklan, Trans. Geol. Soc. Pa., Vol. i, p. 105. 

 Brongniartia platycephala*, Eaton, Geol. Textbook, 1832, pag. 32, pi. 2, fig. 20. 

 Ogygia latissima, Silliman, Am. Journal of Science, Vol. 20, p. 136. 



Homalonotus delphinocephalus, Murchison, Sil. System, 1839, pag. 651, pi. 7 bis, fig. 1 o, h. 

 H. ahrendi, Rcemer, Verst. des Harzgebirges, 1843, p. 39, t. xi, f. 5. 

 U. delphinocephalus, Hall, Geol. Rep. 4th Dist. N. York, 1843, p. 103. 



— — De Verneuil, Note sur le parallelisme, etc., 1847, p. 47. 



H. atlas, giganteus and herculaneus, Castelnau, Essai sur le Syst. Sil. d'Amerique septen- 

 trionale. 



Head ovate or subtriangular, arcuate at the base, more or less convex in the middle and 

 depressed in front, often subacute at the extremity ; glabella scarcely defined ; margin around 

 the front often elevated ; posterior margin of buckler marked by a strong continuous ridge or 

 pseudo-articulation ; facial suture in front, parallel and coincident with, or slightly within, the 

 flexure of the margin, passing thence obliquely through the eye, and, turning, comes to the 

 margin a little above the posterior angle ; articulations of the body thirteen ; central lobe or 

 axis broad, scarcely defined, the articulations continuous, with a slight bend along the line of 

 separation ; articulations with a broad deep groove or furrow running nearer to the anterior 

 margin, and continuing across the axis and into the lateral lobes to the point where they bend 

 downwards, below which it is a sharp impressed line ; lateral articulations falciform, flattened, 

 much expanded and rounded at the extremities, anterior margin arcuate ; caudal shield trian- 

 gular, acute and acuminate at the extremity ; middle lobe with eleven to thirteen articulations, 

 lateral lobes with seven to nine articulations ; surface papulose-granulate or scabrous. 



This species was first named by Green, from a specimen much smaller than the ordinary 

 size, and his figure and cast have therefore proved unsatisfactory in the identification of the 

 * larger and more common specimens. Our specimens are unquestionably identical with the 

 species figured by Murchison from the Wenlock limestone. This species and the Homalonotus 

 (^Dipleura) dekayii have been most strangely confounded ; arising, no doubt, in the first place, 

 from the unfortunate mistakes in regard to localities and consequent geological position. 

 Although occurring at Rochester, Lockport and other places in the same rock, yet I have 

 never seen a perfect individual from any place except the former ; and it was never known at 

 Williamsville, the locality cited by Green, and quoted by European authors. On the other 

 hand, Dipleura dekayii of Green is cited as not uncommon at Lockport, while in fact it 

 never occurs in that place, nor in the same rock. On the authority of Green, however, spe- 

 cimens from Lockport have been labelled Dipleura dekayii, till the error has been widely 



*I have had an opportunity of examining the imperfect specimen from which the figure and description of Eaton 

 were made, and there is no doubt of the identity with the species under consideration. 



