TRENTON LlMESTONEi 
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43. 1. ISOTELUS GIGAS.* 
Pl. LX. Figs. 7 a - i; Pl. LXI. Figs. 3a- m, & 4 a b c ; Pl. LXII. Figs. 1 a b c, & 2; and Pl. LXlII. 
Asaphus platycephalus. Stokes, Trans. Geol. Soc. London, New series, 1822, Vol. i, p. 208, pl. 27. 
Isotelus gigas, and I. planus. De Kay, 1824, Ann. Lyc. Nat. Hist. New-York, Vol. i, pag. 176, pl. 13, fig. 1,2; 
and pl. 13, fig. 1, 2. 
Isotelus gigas. Green, Monograph, 1832, p. 67 & 68. 
Isotelus cyclops, I. megalops, and 7. stegops. Id. Ib. p. 69, 70 & 71. 
Asaphus platycephalus. Buckland, Bridgw. Treatise, Vol. ii, pag. 76, pl. 45, fig. 12. 
Isotelus gigas. Vanuxem, Geol. Rep. New-York, 1842, pag. 46, fig. 1. 
— — Emmons, Geol. Rep. New-York, 1842, pag. 389, fig. 1. 
Isotelus gigas, and I. planus. Portlock, Geol. Rep. Londonderry, 1843, pag. 295, pl. 7, figs. 1, 2 & 3. 
Compare Asaphus cornigerus, Brongniart, Crust, fossiles, 1822, pag. 18, pl. 4, fig. 10 (not pl. 2, fig. 1 a, b). 
Brongniartia isotela, Eaton, Geol. Text-Book, 1832, pl. 2, fig. 19. 
Asaphus powisii, Murchison, Sil. System, 1839, pag. 661, pl. 23, fig. 9. 
Isotelus powisii, Portlock, Op. cit. pag. 297, pl. 6, fig. 1. 
Isotelus ovatus, I. intermedins, I. sclerops, and other figures. Portlock, Op. cit. pag. 297 - 299, plates 
6, 8, 9 & 10. 
Isotelus megistos, Locke, Am. Jour. Science, 1842, Vol. xlii, p. 366. 
— — Trans. Assoc. Am. Geologists and Naturalists, 1843, Vol. 1, pag. 221, pl. 6. 
General figure oval-oblong, with the sides rather straight; buckler in the form of u a 
spherical triangle,” obtuse or more or less rounded at the posterior extremities ; cephalic 
shield convex in the middle, rapidly descending in front and at the sides, margined by a 
narrow elevated rim or border; eyes sublimate, prominent, u subpedunculated,” strongly 
supported on the inner and concave side by a projection of the glabella; facial suture 
continuing from the centre of the front, nearly parallel to the margin, until in a line with 
the eye, when it turns backward, and leaving the eye upon the maxillary portion, turns 
outwards and backwards, coming out at the base of the shield distant from the angle ; 
thorax with eight articulations, the middle lobe about once and a half the breadth of the 
lateral lobes, the longitudinal grooves continued slightly into the buckler, and more dis¬ 
tinctly into the caudal shield ; segments of the middle lobe fiat above, those of the lateral 
lobes with a groove on their upper surface, extending nearly half way to their extremities ; 
caudal shield of nearly the same form as the buckler, presenting externally some evidence 
of a trilobate character, sharpened at its upper lateral angles, and having a single trans¬ 
verse groove on each of the lateral lobes extending from the axis more than half way to 
.the margin ; entire surface finely punctulated. 
When the crust of the buckler is removed, a narrow shallow groove is visible at the base. 
In young specimens the caudal extremity is more pointed, and presents the marks of eight 
anchylosed articulations ; in older specimens, these increase in number, but the external 
* I have adopted the name of De Kay, though aware that the name of Stokes has precedence in point of time. If 
it be proved that Isotelus cannot be separated from Asaphus by any important characters, we must abandon the 
genus; but at present it is well known to all American naturalists, and a change of name would not be attended with 
any beneficial results. 
