RAYMOND: ISOTELUS GIGAS DEKAY. 249 



N. J., 1902, 3, pi. 14, fig. 5, cop}' of preceding (= Isolclus iowensis). 

 Grabau and Shimer, N. A. index fossils, 1910, 2, p. 293, fig. 1()02 (= Iso- 

 ielus latus Raymond). 



Asaphus {Isotelus) gigas Salter, Mem. Geol. surv.. Unit. Kingdom, IStil, dec. 

 11, pi. 3; Mon. Brit. Silur. tril., 18G5, p. 161, pi. 24, fig. i-5; ? pi. 25, 

 fig. 1. Reed, Lower Silur. tril. Girvan dist., 1904, p. 45, pi. 7, fig. 1. 



Asaphus gigas ? Nicholson and Etheridg(\ Mon. Silur. foss. Girvan di.st., 1879, 

 fa.sc. 2, p. 153, pi. 10, fig. 18, 19; Mem. Geol. surv. Silur. rocks Brit., 1899, 

 1, Scotland, p. 509, 513 514 (= Isotelus instabilis Reed). 



Asaphus platycephalus Nieszkowski, Arch, naturk. Liv.- Est.- und Kurl., 1857, 

 ser. 1, p. 551, pi. 1 ( = Isotelus remigerus Eichvvald and /. 7-obustus 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, pis. 31, 32 (= Isotelus 

 latus Raymond). 



From the above synonymy, text-book and catalogue references have 

 for the most part been omitted, as have also references where there is 

 neither description nor original figure. So far as can be judged from 

 the pubUshed figures and descriptions, none of the foreign specimens 

 referred to this species really belong to it, and most of those which 

 have at one time or another been so referred are now known by other 

 names. 



Ontogeny, 



In a recent paper (Ann. Carnegie mus., 1910, 7, p. 53), Mr. Narra- 

 way and the writer summarized the ontogeny of Isotelus 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 pygidiura 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 fiat, without the 

 convex body of the hypostoma of either Isoteloides or Asaphus. 



The smallest specimen (No. 45) showing the dorsal aspect of the 



