26 VISIBLE STRUCTURE 



axis and lateral lobes ; for, considering the entire absence of remains of the crust itself, 

 there can be no question that in the impressions of both species we see the impression 

 of the internal surface of the shell. 



As little can I agree with Emmerich, who adopts the views of Audouin, that the 

 lateral lobes consisted of two pieces, which correspond with the episternon and epimeron in 

 the thorax of insects. There is not only no reason whatever for such a conjecture, but a 

 decisive argument can be brought against it by referring to the fact, that the plates alluded 

 to in insects are always portions of the crust, which encases the axis itself; while here 

 they would appear as lateral projections, without forming part of the covering of the axis. 

 Where there are no especial parts of the skeleton at each separate ring, as is the case 

 with respect to the Trilobites, those names should not be applied which refer exclusively 

 to such particular parts of the skeleton ; nor should they even be made use of in the way 

 of analogy if they were introduced to describe parts differently situated, for such a mode 

 of proceeding will cause the utmost confusion.* In fact, I must repeat that the lateral 

 lobes are nothing more than lateral continuations of the crust covering the rings of the 

 body, that they are incapable of independent motion, and that they serve no other purpose 

 than that of protecting the delicately constructed feet situated beneath them. On this 

 subject I shall proceed to enlarge in the following chapter, and then endeavour to reproduce 

 the absent organs of the Trilobites from the analogy of living forms of Crustacea ; but 

 it still remains to be mentioned witli regard to the central parts of the body, (tlie real 

 body rings,) that each generally consists of two semicircular protuberances, situated one 

 behind another, of which the anterior and smaller one is hidden beneath the projecting 

 margin of the preceding ring when the body is in a stretched position, but which can 

 be very distinctly seen when the body is curved or doubled together. At the end of the 

 furrow, which separates the two protuberances, there is seen at each side the articular 

 cavity, which is nearly circular in the species of Fhacops and Cali/me»e, and rather transverse 

 in Asaphtis and Illcenus ; concerning the use of this I have already given the necessary 

 information. We miss it entirely in all species and fragments deficient in the horny shell, 

 since both parts, that is, the articulating process and the corresponding articular cavity, 

 merely belong to the horny coverings. The transverse furrow of the central body, by which 

 the anterior protuberance of each ring is separated from the posterior one, in most cases 

 partially extends itself to the lateral lobes, and only disappears at the place where the 

 latter bend themselves downwards by approaching to the posterior margin of the lobe, and 



* Audouin, in his well-known work on the skeleton of insects, (Annal. des Scienc. Natur., Prem. 

 Ser., torn, i, 1824,) calls that part of the skeleton epimeron, which is situated between the freely move- 

 able paunch (?) and the back plate ; episternon, on the other hand, he terms that part of the skeleton 

 situated between the sternum itself and the back plate. Among the Trilobites, we find the only 

 instance of these divisions of the external skeleton into separate pieces, in the head ; in all the other 

 parts of the body there is nothing of the kind. This circumstance is a most remarkable one ; and it 

 is without any an.alogy in living Crustacea, the shell of which always forms a continuation at the iu- 

 di\idual rings, and never consists of separated pieces, connected by sutures, not even when it distinctly 

 covers several rings. Dalman has already recognized and publislicd (Palaead. p. 13) an account of this 

 exception from the general rule, that the skeleton parts of living Crustacea never have sutures ; and I 

 must once more particularly enumerate it as a most singular character of the Trilobites, although at 

 the same time it is necessary to observe that the peculiarity which distinguishes the head-crust of these 

 animals does not appear in the other rings of the crust. 



