44 AFFINITY OF THE TRILOBITES 



abdomen, they must have been, as in the other Crustacea, directly attached to the hard 

 upper shell, in order that there should be a sufficiently solid basis for the organs of locomo- 

 tion. But we may well inquire how all such solid girdles inclosing the abdomen could, if 

 they ever existed, have been broken off with such perfect regularity that they have not even 

 left a single vestige. Such an assumption is beyond the bounds of probability, and yet we 

 must suppose this to have been the case, if we suppose that the abdominal surface of Trilobites 

 was provided with a solid covering like that of the back, and the assumption is equall}' 

 necessary if we believe these animals to have had hard horny extremities, since such 

 extremities are never found in the Articulata unless accompanied by a solid thorax. 



Considering then that all traces of the extremities are absent, we may be permitted to 

 assume that the feet of Trilobites were too soft and delicate to have left even their 

 impressions, and this is precisely what might have been expected, if my view of the affinity 

 of these animals to the Phyllopoda is correct ; but although this, and other reasons already 

 given, might of themselves be considered sufficient to establish the fact, it may be proved yet 

 more distinctly by referring to the power possessed by most species of the extinct family to 

 double themselves up, a faculty often exhibited in the specimens found fossil which have 

 been preserved in this form. In performing this evolution the animals in question arched 

 the back, and bringing the caudal shield in contact with the under part of the head, 

 concealed all the abdominal surface beneath the hard horny coating of the upper side. 

 Now there is no imaginable reason why the animal should have been endowed with a power 

 of thus rolling itself into a ball, if the under side of the body were defended with a horny 

 or solid shield ; but we can well understand the importance and meaning of it, if the under 

 side Were, as we suppose, undefended, for it is then a simple effort of nature to protect 

 these soft and vulnerable parts against external violence. It may, indeed, be said that 

 some genera, such as Odontuphura, Oc/ijc/ia, Olenus, &c., were not endowed with this faculty, 

 and that therefore no general inference can be drawn, but this, in point of fact, is not a valid 

 objection, since it appears, from the frequent absence of all remains of the hard covering in 

 the case of these genera, that their shells must have been softer and more tender than the 

 shells of other Trilobites, and I think there is reason for concluding that this was the case, 

 from certain specimens which I have obsei'ved and examined of Olcnus gibhosus from the 

 alum slate of Andrarum. In this case, if the shell were thus thin and tender, as in Apm, the 

 power of doubling the body together into a ball would have been useless, as it would offer 

 no protection. Indeed, in these cases, the lateral lobes are so constructed that undefended 

 spaces would have been left if the body had been doubled, so that no advantage would 

 have been gained. We may therefore conclude that even in those cases where the body 

 does not appear to have been capable of being doubled up, the feet were still soft^ and we 

 may venture to assert that in the OlenidcB the impressions of the feet themselves would 

 have been found, if they had been as hard as or harder than the soft covering of the body 

 of these animals. 



Proceeding, however, with the comparison, let us now consider the structure of the 

 extremities of the living PhyllojMda. They exhibit, as we have already seen, only one 

 principal type, modified with regard to the arrangement of the gills, and this modification 

 depends on the presence or absence of a horny or calcareous covering of the body. Among 

 the Trilobites whose body was provided with a shell on the upper surface, and which were 



