OF THE TRILOBITES. 25 



lobes consisted (exactly as in the already described flattened extensions of the cephalic and 

 caudal shield) of two layers, between which there was a thin layer of the substance of the 

 body, and these were, on the external, open, upper surface, partly smooth, partly granulated ; 

 but on the lower hidden surface furnished with parallel striee. This is distinctly seen in the 

 fragments of Amplius and IUwiiks, in which both coverings of the lateral lobes arc generally 

 preserved, showing that the intermediate layer at the upper and internal parts of the lateral 

 lobes was thicker than at the lower part, (which is curved forwards,) where each lobe 

 terminated in an acute angle towards the external and upper part. On the other hand, it 

 formed a broader rounded facing. If I appreciate these impressions correctly, I should 

 say that the internal horizontal part of each lateral lobe must have been in immediate 

 connexion with its neighbour, and that this whole region of the body has participated 

 in the protection of the fleshy muscular layer, situated beneath the arched central part, 

 or may even have been partly the support of this muscular portion. This is probable, 

 since in all the specimens, even those that are rolled up, these regions of the lateral lobes 

 are not projected one al)ove another, but are at the usual distance from each other ; and 

 I think I can perceive a kind of articulation in the anterior angle of each posterior ring, 

 ■where the external part of the lateral lobes bends downwards. Such an articulation 

 certainly exists at the place where the central arched part of each ring meets the 

 lateral lobes, not, however, between this part and its lateral lobe, but between the central 

 arched body rings themselves. At this spot, indeed, may be observed a strong hemispherical 

 articulated head, immediately before the open posterior margin of the ring at its lower 

 surface ; and this head fits into an articular cavity, formed to fit it in the succeeding ring. 

 The latter exists at the anterior margin of the caudal shield, and is distinctly represented 

 in Table V, Fig. 4. The first pair of articular processes occurs, however, at the posterior 

 margin of the cephalic shield, and thus each segment has a pair of articular cavities on 

 its upper side anteriorly, at the junction of all the rings with the preceding covered 

 margin ; and on the other hand, at its lower free side, which partially projects over the 

 succeeding ring, it has a pair of hemispherical articular processes. These and the cavities 

 may be very distinctly recognized in the larger specimens of the species of Phacops, whose 

 horny coat has been lost ; the fractured «irticular prominences being usually still found in 

 the articular cavities beneath them. There can also be no doubt, from the analogy of living 

 Articulata, that besides this a soft articular membrane connected the margins of the rings, 

 situated opposite to each other ; but in all other respects, each separate ring was complete 

 in itself, its lateral lobes being immediate continuations of the central arched principal 

 portion, and nowhere connected with the latter by means of sutures. It is true indeed 

 that deep impressions are found at the sides of the thorax in well-preserved specimens 

 of Ogyyia Buchn and Conocev/iahts Sitheri, separating the latei'al lobes of each individual 

 ring from its axis ; but I should not be inclined to look upon these as sutures, which 

 Emmerich declares them to be, because nothing of the kind is found in the other Trilobites ; 

 and there are no apparent means b)' which these lobes could have been moved (as their 

 mode of insertion indicates that they might have been), since they could only have had 

 a very slight muscular layer on account of the thinness of the lateral lobes. I believe, 

 therefore, that the suture-like furrow alluded to does not indicate a suture, but originates 

 from an acute angle, which projected here at the internal surface of the crust, between the 



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