482 



PB0CBRDIN08 OF THR GKOLOQICAL 80CIKTT. [May 1 1 , 



above noticed waa split. Portions of the lower niarg;in» ol' the head 

 and tail, and the extremities of some of the pleune, reraiiinod stick- 

 ing in the stone. It can also he proved by polished sections throuifh 

 the head and tail of any well-preserved specimen. Such seciions 

 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 (PI. XXXI. figs. 2, 3, 4). The plcurie 

 have tdso a doublure, which extends upwards, nearly halfway to the 

 median Icbe of the body. In conse^iucnce of this structure the ex- 

 tremities of the pleurae are hollow, exactly like those of a lobster. 



In Lhnulus a similar doublure occurs ; ajid we can see thrae that 

 it is continuous with the thin membranous crust which eovers the 

 underside of the body and bears the limbs. Eetwccn the sternum 

 of Limulus, with its load of ponderous legs, and the doublure there 

 is no connexion, all round, excej)t this fragile membrane. In con- 

 sequence of this structui'e it often comes away with all its appen- 

 dages, leaving nothing of the animal except its huge earapuce, 

 pygidium, and telson. Specimens of this great crab in this condi- 

 tion arc oommon in museums. 



In the geiuis Asaphm, and, no doubt, iu all other Trilobites, the 

 doiiblure is, as in these ira]»erfecti specimens of Lhtmlns, only the 

 remains of the integument M'hieh covered th<* und(;rside and sup- 

 ported the sternum. These two genom, however, differ widely in 

 other respects. 



The doul^luro of A. platycephalus was figured by Dr. Bigsby so 

 long ago as 1823, in the Geological Transactions, 2nd series, vol. i. 

 pi. xxvii. fig. 1 e, among the illustrationf of his paper " On the (ieo- 

 graphy and Geology of Lake Huron." The figure shows a section 

 through the doublure on the right side, just in front of a line dravvii 

 across the head through the centres of the eyes. In the description 

 of the figure the true character of the part in (juestion is recog- 

 nized, by the remark that '* the shelly crust of the tmder side Joins 

 the upper at the sides.-' It is also shown in fig. 1 b, on the same 

 plate, which represents the underside of the same specimen, with 

 the hypostoma in place*. In that paper this now famous Trilobite 



* Tills is the second hypostoma ever figured. Barrande, in his great work on 

 the Trilobitesof Bohemia, comraonces the history of the organ in question, thus: — 



" A.. Iktnn{e» Hisforiques. 



" 1821. Leplus ancien liypostome connu, est Sgur6 et d<5crit par Wahlenberg, 

 sous le nom de EniomostracUes bi'cephalm (Nov. Act. Soo. Sci. Upsal, viii. '67, 

 pi. i. flg. 6). 



" 18'22. Ch. Stokes d^couvre sous la t^te d'Asaphus platr/ccphalus (^Tsofelus 

 gigas, De K.) une pittoe cpustacee. plac6p a I'entree dti restomac; et il lu deerit 

 d ms les Transact. Geol. (nouv. ser. i. 208, pi. 27). 



" La mfime ann^, le savant Am^rictiin D*' Kay deerit et repr^sente lo m'^me 

 appareil que nous retrouvons figure par Buckland dans les Bridgew. Treatises, 

 en 1837." (Barmnde, Systenic Silurien tfec. vol. i. p. 1.54.) 



There is a difficulty about the nomenclature of tliis Trilobite, owing, in part, 

 to some uncertainty as to the true dates of publication. In the later rejKjrta of 

 our survey we have adopted the name given to it by Stokes, while most American 

 authors call it either Asu jhas gigns or Im)telus gigas. Dr. Bigsby'e paper woe 



