OF THE TRILOBITES, 23 



so great a mobility in those Articulata, whose shell pieces are connected by sutures ; on the 

 contrary, the mobility of the plates upon one another is always very slight, and a considerable 

 distance of the margins of the suture from one another is impossible, if it were only because 

 a soft membrane arises from the internal margin of the sutures, connecting intimately both 

 tlie suture margins. Owing to this, the elements of the skeleton of the liighest Articulata 

 can at the utmost be only bent towards each other, never being separated from one another 

 to any considerable extent. The facial suture of the Trilobitcs probably likewise admitted of 

 such an easy bending of the lateral shields towards the central shield, and might be intended 

 for the purpose of arching the space beneath the cephalic shield somewhat more during the 

 contraction, so that the requisite height might be gained for the feet, which were then hidden 

 beneath the cephalic and caudal shields. Indeed the intimate union of the lateral lobes of the 

 segment of the trunk in one section at the posterior angle of the scuta temporalia testifies that 

 the object was to conceal all the lower parts as much as possible beneath the head-crust 

 during the rolling up of the Trilobitcs.* Such a section is found at the lower side of the angle 

 alluded to, immediately behind the external margin ; it is particularly distinct in the genera 

 Asapltus and lUcemm, visibly sharpens the margin, which before that point is thick, broad, 

 and rounded off, and thereby causes a cavity in the margin itself, running parallel with the 

 acute angle, the cavity being intended for the reception of the lower end of the last lateral 

 lobe, situated before the caudal shield, when the Trilobite was rolled up. The axis, 

 around which the animal doubles itself, is situated very near to the locality where 

 the two furrows, which run parallel with the lateral and posterior margin of the scuta 

 temporalia, meet together before the posterior angle, and the lowest ends of the lateral lobes 

 of the joints of the trunk also usually conceal themselves to that extent beneath the 

 cephalic shield. The excavation behind the margin before described serves, therefore, for 

 their reception, and indicates that a Trilobite possessing it had the power of doubling 

 itself together. On the other hand, however, it will not do very well to infer from the 

 absence of the section, that such a Trilobite could not have rolled itself up. I certainly have 

 always looked in vain for it in all the Oleneides, nor have I ever perceived any traces 

 of the capability of doubling themselves in these Trilobitcs ; but I could quite as little 

 discover that section in Phacops and Calijmeno. 



SECTION IX. 



We have now still to investigate those remains of the existing parts of the cephalic 

 crust, which have been observed on its lower surface behind the margin, and evidently in 

 front of the mouth. The first who observed this region of the head in Olenus Tessini, Dalm., 

 was Wahlenberg ; he took it, however, for the impression of the upper side of another 

 species, and described it as Entomostracites buccphalus (p. 37, 10, Table I, Fig. 6, of his work). 

 Subsequent to him, the same region was observed and represented by Stokes in Isoteles gigas 

 (his AsupJi. plufi/ccj)Jialus), and by Eichwaldt in Asaph us eapansus, Wahl, (his CryjAonymus 

 Panderi), but they were not correctly observed by them. The same may be said of Pander, 



> Tabic ^'I, Fig. 8, // 



