14 VISIBLE STRUCTURE 



SECTION III. 



My observations on this subject must be preceded by the explanation, that the real 

 shell has by no means been preserved in all the Trilobites, but that a great part of their 

 remains consists merely of impressions from the shell. This is the case in all the Trilobites 

 of the grauwacke and of the clayslate, therefore particularly in the OJenida ; undoubted 

 remains of the shell itself are first found in the specimens from the alum slate, and the 

 same is more or less perfectly preserved in most of the individuals inclosed in the transition 

 limestone.* In individuals from this rock, especially in such as are found as loose 

 stones in many localities of Northern Germany, and which are already perfectly freed 

 from the limestone that formerly surrounded them, we see most distinctly that the shell con- 

 sisted of two layers, of which the external one extended itself over the lower, thicker, darker 

 layer as a very thin, and generally clear coloured, coat. This fine coat is closely covered with 

 small uneven tubercles, or is granulated on its whole external surface, and has therefore quite 

 the appearance of the horny shell of our river crawfish, especially at the claws. These granu- 

 lations were so slight over most parts of the body, that they left no trace at all in the second 

 or lower layer of the shell ; but their presence in the more elevated portions, as, for instance, in 

 the arched anterior portion of the head, and on the rings of the body, betrays itself, even when 

 the upper membrane is wanting, by light but larger tubercles, which cover these spots. They 

 attain their greatest development in the CaJijinene variolaris, which derives its name from 

 them, but they are likewise not wanting in the Dudley Trilobites {Calymene Bhnnenbaclni). It 

 is only in these, and in the smaller specimens (var. jmJcheUa), that I have hitherto been 

 able to observe the external layer with its granulations in a well-preserved state ; the upper 

 layer is almost always wanting in the granulated species of Pliacops ; the general granu- 

 lation, therefore, can only be inferred from the existence of those larger granulations of 

 the lower layer of the shell. The granulated surface, however, seems to have been a general 

 quality of the group in the two genera Cali/meiie and Pliacops, and seems to belong to all their 

 species. Most published figures of /"//«fo/)« confirm this opinion; the granulation in the genus 

 Calymene has generally been overlooked, because it is here much finer and slighter, and 

 because it is usually only recognized on the upper membrane itself. If, however, the 

 second layer of the shell has likewise been cast ofi', and if the impression of the interior of 

 the shell of the Trilobite only is existing, those indistinct coarser traces of granulation are, 

 as a matter of course, also wanting, and the surface appears to be smooth. This is not only 

 veryfrequently the case with regard to Calymene Blnmenhackii , but also very often with 

 Pliacops latifrons, and with regard to the latter has given rise to the enumeration of several 

 species (C. latifrons, and C. Schlotheimii, Bronn). P. iwofuherans, and all the species of 

 this genus which are described as smooth, seem to originate from those individuals the 

 membranes of which have been cast off. 



* These remarks ^Tere intended, no doubt, bj^ the author, to refer chiefly to the distribution of 

 Trilobites in the rocks in his own neighbourhood. The actual shell of these animals is found frequently 

 in the Siliu'ian limestones in England, and sometimes in the Caradoc sandstone, the oldest rock in 

 ^vhich they appear. The shell is found also in the Devonian and carboniferous limestones. — Eng. Ed. 



