OP THE TRILOBITES. 15 



SECTION IV. 



The upper membrane just described seems only to be a peculiarity occurring in the genera 

 mentioned, and of some others {Brontius, Odontopleura, Jloiiialoiiotus), but wanting in most of 

 the Trilobites. Not even the slightest trace of a more delicate layer, capable of being 

 thrown off, can ever be discovered on the surface of the shell of perfectly well-preserved 

 remains of the genera Asapliiis and Illanus, indicating a different quality of the horny 

 covering of these genera. In individuals whose external surface is in some places not at all 

 injured, I perceive, on the other hand, fine deeply cut lines, which run pretty much in the 

 same manner as the furrows in the palm of the human hand, but are situated more remote 

 from one another, and have fine punctured dots between them. I observed this character of 

 the surface-membrane in Calymene and Phacops, but most distinctly in Asaphus expanms, var. 

 cornigerus, upon the arched anterior portion of the head, and at the most elevated parts 

 of the rings of the body, and often exactly at those points where the granulation is 

 most perfect ; on the other hand, I noticed those fine points in greater number and in a 

 closer position on the lateral portions of the cephalic shield, on the lateral lobes of the rings 

 of the body, and on the caudal shield, but in those places they are only single, coarse, rather 

 elevated diagonal lines, which, however, are distributed in a tolerably symmetrical manner. 

 This formation likewise meets with its analogue in living Crustacea, and may be found in the 

 thorax of the lobster, particularly as regards the punctures. 



These granulations and punctures, however, only exist on that surface of the shell 

 which is at the superior side of the animal, for the inferior surface, as far it has been 

 preserved, has a diS'erent structure. It was likewise covered by a peculiar, but always 

 thinner, horny membrane, which, however, gradually became more delicate the nearer it 

 approached the middle, being everywhere separated from the upper side of the shield by 

 a layer of muscle, and itself consisting of a softer structure. These statements may be 

 verified by observations ; and, as one reason in support of them, I may state that we always find 

 in the remains of Trilobites, in which both layers of the shell are existing, that there is a layer 

 of rock between them, which indicates their distance from one another. As another reason, 

 we may state that we observe at once the thickness of the petrified shell by such layer of stone, 

 and perceive that the lower layer is thinner than the upper. For the better understanding of 

 these proportions, I beg to refer my readers to the illustration of the large Asapjhts shield, 

 which I have given in Plate V, Fig. 4. This shield is still covered on its left side by its old 

 petrified shell, furnished with its natural surface, and therefore only exhibits a tolerably 

 well-defined system of parallel striae at that part of the anterior margin which was overlapped by 

 the lateral lobes of the last thoracic ring. A sharp broken edge, which at first runs along 

 the whole length of the middle of the abdomen, and then turns towards the left, indicates 

 the limit of the broken shell. That which is still visible towards the right is only the 

 impression of the shell on that part of tlie stone which penetrated into the shield of the 

 abdomen. From this part, however, a considerable piece is broken off at the posterior 

 margin, and there is not only visible a part of the lower shell, but also its impres- 

 sion into the stone situate beneath it, at those points where the shell itself is wanting. 

 This accidental quality of the shield proves distinctly that the lower surface of the 



