482 PROCEEDINGS OF THE GEOLOGICAL SOCINTY. [May 11, 
above noticed was split. Portions of the lower margins of the head 
and tail, and the extremities of some of the pleure, remained stick- 
ing in the stone. It can also be proved by polished sections through 
the head and tail of any well-preserved specimen. Such sections 
usually show that a portion of the crust, called the “ doublure ” by 
Barrande, all round the margin is folded under and reflected upwards, 
ending in a free thin edge (Pl. XXXI. figs. 2,3, 4). The pleure 
have also a doublure, which extends upwards, nearly halfway to the 
median lobe of the body. In consequence of this structure the ex- 
tremities of the pleure are hollow, exactly like those of a lobster. 
In Limulus a similar doublure occurs; and we can see there that 
it is continuous with the thin membranous crust which covers the 
underside of the body and bears the limbs. Between the sternum 
of Limulus, with its load of ponderous legs, and the doublure there 
is no connexion, all round, except this fragile membrane. In con- 
sequence of this structure it often comes away with all its appen- 
dages, leaving nothing of the animal except its huge carapace, 
pygidium, and telson. Specimens of this great crab in this condi- 
tion are common in museums. 
In the genus Asaphus, and, no doubt, in all other Trilobites, the 
doublure is, as in these imperfect specimens of Jimulus, only the 
remains of the integument which covered the underside and sup- 
ported the sternum. These two genera, however, differ widely in 
other respects. 
The doublure of A. platycephalus was figured by Dr. Bigsby so 
long ago as 1823, in the Geological Transactions, 2nd series, vol. 1. 
pl. xxvu. fig. 1 ¢, among the illustrations of his paper ‘‘ On the Geo- 
graphy and Geology of Lake Huron.” The figure shows a section 
through the doublure on the right side, just in front of a line drawn 
across the head through the centres of the eyes. In the description 
of the figure the true character of the part in question is recog- 
nized, by the remark that “ the shelly crust of the under side joins 
the upper at the sides.” Itis also shown in fig. 1 6, on the same 
plate, which represents the underside of the same specimen, with 
the hypostoma in place*. In that paper this now famous Trilobite 
* This is the second hypostoma ever figured. Barrande, in his great work on 
the Trilobites of Bohemia, commences the history of the organ in question, thus:— 
“A. Données Historiques. 
“1821. Le plus ancien hypostome connu, est figuré et décrit par Wahlenberg, 
sous le nom de Hntomostracites bucephalus (Noy. Act. Soc. Sci. Upsal. viii. 37, 
pl. 1. fig. 6). 
ae: 1882, Bis Stokes découvre sous la téte d’Asaphus platycephalus (Isotelus 
gigas, De K.) une piéce crustacée, placée a Ventrée de Vestomac; et il la décrit 
dans les Transact. Géol. (nouv. sér. i. 208, pl. 27). 
“La méme année, le savant Américain De Kay décrit et représente le méme 
appareil que nous retrouvons figuré par Buckland dans les Lridgew. Treatises, 
en 1837.” (Barrande, Systéme Silurien &c. vol. i. p. 154.) 
There is a difficulty about the nomenclature of this Trilobite, owing, in part, 
to some uncertainty as to the true dates of publication. In the later reports of 
our survey we have adopted the name given to it by Stokes, while most American 
authors call it either Asaphas gigas or Isotelus gigas. Dr. Bigsby’s paper was 
