24 VISIBLE STRUCTURE 



whose representation is certainly more particular, but without any correct interpretation of 

 what these parts really are.* Sars was the first who recognized them for what they are, 

 namely, for the lower protuberance of the head before the mouth, and described them as 

 such in the genera Illa'uun and xhaphis ('Isis,' 1835, p. 340, Table IX). I myself have only 

 hitherto observed this region pei'fectly in Phacnps and Parado^rides ; in Amphus and Illanxs, 

 however, I have discovered them so distinctly, that I cannot doubt their existence, and the 

 correctness of those former representations. The great similarity in the figures of the four 

 authors, who, however, were not acquainted with the works of their predecessors, also speaks 

 in favour of this opinion. The following is the structure observed. 



A moderately arched protuberance, which in size and circumference corresponds pretty 

 nearly with the most anterior portion of the head on the ujjper side, exhibits itself immediately 

 behind the thickened anterior margin of the cephalic shield, that part which Pander terms 

 lateral gill. It was intimately connected with the anterior margin of the head, and has 

 certainly not been moveable at pleasure, as Pander supposes, in consequence of its isolated 

 position in some individuals. {Vide Table IV, B, Figs. 3, 4, of his w^ork.) From the 

 anterior part it extends itself with a pair of lateral lobes, which are more or less distinctly 

 separated from the central lobe, along the margin indicated, towards the external part, and 

 there terminates in a long, more pointed, less arched projection. Towards the posterior 

 part in Puradoxides there is a rather protuberant margin, curved outwards, and before it a 

 considerable oblique excavation. This margin in Asaplms and Illceiuis, on the other hand, is 

 deeply notched and strongly double lobed. In all three are exhibited on the whole surface 

 the same indented concentric lines, which cover the lower surface of all parts of the shell. 

 Sars certainly represented such lines only on the lateral lobes, but I have found them 

 everywhere on the whole surface in P. bohemiciis {Enf. hiicrpJialiis, Wahl.), but certainly 

 slighter in the centre, and therefore I suppose that Sars must have overlooked them. I 

 have represented this region of P. boliemiciis on Table I, Fig. 7, and have availed myself of 

 ■Sars' figures of Asaplms in my drawing, Table VI, Fig. 8. 



There can now scarcely be a doubt that this region is the ordinary enlargement before 

 the mouth, which we perceive in the Plii/llojiodes, and which is usually called hyposioma. 

 This certainly testifies as decidedly in favour of the affinity of the two groups, as it negatives 

 the affinity with the Isopudes. But of this hereafter. 



SECTION X. 



The tliornx or bodu of the Trilobites, to the description of which we now proceed, 

 consists of a number of homogeneous rings, of which every one likewise possesses a horny 

 crust. The latter has, as on the cephalic and caudal shield, lateral freely projecting 

 lobes {pleurcB) at each ring, which are readily distinguished by their flattened and ge- 

 nerally incurved form, from the uniformly arched semi-cylindrical body. These lateral 



* Pander supposed that these, as ^ell as the lateral margins of the head that are turned over, were 

 gills ; and terms the hitter lateral gills, the central swelling bcliind the anterior margin central or lower 

 gill, and even believes that there were respiratory organs iu the swelling before the mouth. [Vide pp. 

 134 and 128 of his work.) 



