OF THE SPECIES. 77 



II. 



Trilobites having the poiücr of rolling themselves info a haU. 



The distinctive character of this second, more numerous, and principal group of Trilo- 

 bites is to be sought for in the structure of the lateral lobes of the joints of the body, which 

 at first are continued horizontally, but are afterwards more or less curved vertically downward. 

 At the point of curvature there appears to be a kind of articulation between two successive 

 joints, or, at least, there is a very accurate insertion of one into the other. 



The lobe from this point becomes broader outwards and downwards, ceases to be con- 

 nected with the adjacent ones, and makes a turn, its anterior edge being directed obliquely 

 inwards, to find room by the side of the next preceding. It thus has a surface somewhat 

 turned forwards, obliquely placed, and gradually widening below, extending hence to the 

 posterior margin of the lobe, and only leaving a very small space for the true upper or 

 external side. This space also, the true external surface, gradually diminishes from the 

 point of articulation of the two lobes, and extends to the posterior margin ; it is usually 

 rather strongly arched, and divided by a diagonal furrow, which proceeds from the anterior 

 angle, close beside the rings of the axis, and likewise bends towards the external posterior 

 angle. The anterior sharper margin of this furrow forms at the same time the edge, at 

 which the oblique, but always flat, anterior surface meets with the posterior or upper sur- 

 face. When the animal rolls itself up, the lateral lobes were passed one under another from 

 the point of articulation, each preceding lobe covering the oblique surface of the next so 

 completely, that nothing could be seen of the roUed-up animal except the convex posterior 

 portion. I shall henceforth always call the surface, which is covered during the rolling-up 

 process, the anterior, and, on the other hand, the one which remains externally visible, the 

 external part; the former being manifestly intended to be concealed, since it is usually 

 covered by the parallel punctured furrows which are found in all Trilobites at the lower 

 surface of the shell, where exposed. 



Such punctures are never absent in well-preserved specimens ; but the oblique diagonal 

 furrow on the upper side of the rings, on the other hand, is deficient in some genera {lllanus 

 and Nileus). The boundary between the rings of the axis and the lateral lobes is also more 

 indistinct when such is the case. 



There are, however, other characters also more or less directly connected with the 

 power of the animal to roll itself up. As such, we may enumerate — 



1. The much lai'ger and more projecting eyes, a character which, since it is wanting in 

 the previous gi'oup, has led to the assertion, that most of the members of this group were 

 blind ; I have, however, recognized the eyes in almost all of them (except, indeed, Trinucleus), 

 and thus refuted, I hope satisfactorily, the notion of blind Trilobites. 



2. The tougher nature of the horny membrane. It is at least remarkable that this part 

 is preserved in a fossil state in almost all the members of this group, while in the members 

 of the former group it is only met with when the remains are found in limestones. The 

 Trilobites of this second group, however, occur chiefly in limestone, and the preservation 

 of their shell may therefore be owing to that circumstance. 



