RAYMOND: ISOTELUS GIGAS DEKAY. 249 
N. J., 1902, 3, pl. 14, fig. 5, copy of preceding (= Jsotelus iowensis). 
Grabau and Shimer, N. A. index fossils, 1910, 2, p. 293, fig. 1602 (= Iso- 
telus latus Raymond). 
{saphus (Isotelus) gigas Salter, Mem. Geol. surv., Unit. Kingdom, 1864, dec. 
$1, pli 3; Mon. Brit. Silur. tril., 1865, p. 161, pl. 24, fig. 1-5; ? pl. 25, 
fig.1. Reed, Lower Silur. tril. Girvan dist., 1904, p. 45, pl. 7, fig. 1. 
{saphus gigas ? Nicholson and Etheridge, Mon. Silur. foss. Girvan dist., 1879, 
fasc. 2, p. 153, pl. 10, fig. 18, 19; Mem. Geol. surv. Silur. rocks Brit., 1899, 
1, Scotland, p. 509, 513, 514 (= Jsotelus instabilis Reed). 
{saphus platycephalus Nieszkowski, Arch. naturk. Liv.- Est.- und Kurl., 1857, 
ser. 1, p. 551, pl. 1 (= Jsotelus remigerus Eichwald and I. robustus Roemer, 
teste Schmidt). Billings, Geol. Canada, 1863, p. 184, fig. 183 (= Iso- 
telus latus Raymond); Cat. Silur. foss. Anticosti, 1866, p. 24, fig. 7 (= Iso- 
telus latus Raymond); p. 26, fig. 8b (= Brachyaspis altilis Raymond); 
Quart. journ. Geol. soc. London, 1870, 26, p. 486, pls. 31, 32 (= Jsotelus 
latus Raymond). 
From the above synonymy, text-book and catalogue references have 
or the most part been omitted, as have also references where there is 
1either description nor original figure. So far as can be judged from 
he published figures and descriptions, none of the foreign specimens 
eferred to this species really belong to it, and most of those which 
jave at one time or another been so referred are now known by other 
lames. 
ONTOGENY. 
In a recent paper (Ann. Carnegie mus., 1910, 7, p. 53), Mr. Narra- 
way and the writer summarized the ontogeny of Jsotelus gigas. The 
Walcott collection in the M. C. Z. contains more complete material 
than we then had, and permits a study of the species from the stage 
in which it had a length of only 3 mm. 
‘The smallest specimen (No. 36) in the collection is 3 mm. long, and 
has the same width at the genal angles. It is exposed from the lower 
side, and retains the hypostoma in position. The details of the 
thorax and pygidium are not well shown, but the cephalon occupies 
at least one half the length, and the pygidium is considerably smaller. 
The genal spines are long, extending back nearly to the posterior end 
of the pygidium (Fig. 2). Another specimen, 5 mm. long, is also 
exposed from the lower side, and shows the hypostoma. In both, the 
hypostomas are deeply cleft behind, and rather flat, without the 
convex body of the hypostoma of either Isoteloides or Asaphus. 
The smallest specimen (No. 45) showing the dorsal aspect of the 
