16 VISIBLE STRUCTURE 



shell is furnished with fine parallel lines, as in the covered part of the upper surface at the 

 anterior margin ; that it consists of a horny membrane, thinner than the latter ; and that 

 the distance of the two membranes from each other was much greater in an angle of the 

 lower layer than at the other parts of the whole shield, greater indeed even than at the end 

 of the real abdomen, the lancet-formed point of which — at least in this case — seems to have 

 been flat. For there seems to me no reason for supposing that this part was flattened by 

 external force, and was originally aixhed downwards, since the parallel angle of the lower 

 side is perfectly preserved, and not flattened, which certainly would have been the case if 

 the whole shield had sufi^ercd considerable pressure. 



The lateral lobes of the joints of the body and the whole cephalic shield are formed 

 also like this shield of the abdomen. Thus we may most distinctly convince ourselves, from 

 many fragmentary specimens, that the entire lower surface of the shell of the head was covered 

 in the neighbourhood of the anterior margin with such deeply cut parallel lines, giving 

 that surface the appearance of a regular striation, the furrows of which run parallel 

 with the circumference. The interior as well as the exterior surface of the lateral 

 lobes is formed in a somewhat similar manner, though not entirely so, as far as the latter is 

 covered by the preceding lobe. On this lobe, however, the lines run lengthwise, are not so 

 deep, are frequently furcated, and are in general not so regular as at the cephalic and 

 caudal shield. I have nowhere been able to perceive the dots between these lines of the 

 lower surface, which are situated between them at the upper surface, and which even occur 

 at many places by themselves, without the lines ; they are here, as they are generally, 

 wanting as the granulations at the lower surface of the species of Calymene and PJiacops, in 

 which, however, the cross striae described are as generally found as they are in Asaphus and 

 lllanus. We have not, however, so many opportunities of observing them, since most 

 specimens of these genera, and indeed always the most beautiful ones, are rolled up, so that 

 we are unable to observe any of the parts of the lower surface. 



This is all that I have been able to ascertain with certainty as to the nature of the shell 

 of the Trilobites. I suppose the same structure to be existing in the Olenides as in Asaphus, 

 at least I have been able to convince myself of a similar striation of the lower surface. Genker 

 has also observed the same, and indicated it in his figures, for instance, in Table V, Fig. c d. 



SECTION V. 



Proceeding to the subject of the divisions of the body, and commencing with the 

 consideration of the head as the first, we shall soon observe that this part is encased 

 in a great parabolical semicircular lunate shield {scidiim capitis, cephalic shield), in which 

 the head itself only occupies the central, and, therefore, the more highly arched part. 

 This central part, the head itself, which 1 shall henceforth call head-tubercle {Kopfbxckel — 

 glabella, according to Dalraan), is very distinctly characterized by a furrow round it of 

 greater or lesser depth, is always rather longer than broad, generally broader and thicker 

 at the anterior part, and is there also more highly arched and more strongly projecting. In 

 many instances impressions exhibit themselves on the whole elevation, which proceed from 

 the furrow surrounding it, and which, more or less, penetrate into the head-tubercle, some- 



