38 AFFINITY OF THE TRILOBITES 



which exhibit in association, although incompletely, the peculiarities now found detached 

 and characteristic of very distinct groups. Minute and careful observation has seemed to 

 confirm this view, and exhibits also another important fact, namely, that in proportion to the 

 geological age of the extinct species, the rvmning of the various typical forms into one 

 structure is more marked, and therefore the peculiar and organic individuality and dis- 

 tinctiveness less obvious. 



Such being the case, it will no longer appear strange that the Trilobites, the oldest 

 Articulata that we are acquainted ^^ith, should not exactly correspond with any one group 

 of living Articulata ; and, on the contrary, it would rather be a matter of surprise if such 

 were the case, for their identity with any recent family would overturn those laws which 

 have been developed with so much labour and care in the course of various researches on 

 the organic beings of a previous creation. If therefore any one should assert that the 

 identical representative had been discovered of an animal whose period of existence in a 

 living state was unquestionably very remote, and long antecedent to the commencement of 

 the present era, we might safely assert, wherever the discovery was said to have taken place 

 and even without seeing the supposed representative, that it is not what is thus assumed. 

 No doubt the assertion might be credited by many imperfectly informed persons without due 

 investigation, but the naturalist acquainted with the unity and uniformity of the great plan 

 of Nature, and her method of producing results by laws which are invariable, will not for a 

 moment entertain a belief of the present existence of any species of Trilobites, however 

 comparatively insignificant the creature may be. 



I would by no means assert, however, tliat Trilobites wholl}' deviate from the tvpes of 

 all existing Crustacea, for, on the contrary, many important resemblances are found, which I 

 have already indicated and partially explained. We are even able, by a careful and accurate 

 estimate of these resemblances, and by comparing the details of structure exhibited in the 

 remains handed down to us, to restore those points in the organization that are wanting 

 by considering the affinities with recent forms. It is this task which I now proceed to 

 perform. 



SECTION XVIII. 



In order to proceed with due caution it will be proper, however, first to explain more 

 particularly the peculiarities of that group of Crustacea with which the Trilobites are most 

 nearl}' related, for the purpose of being able to infer thence whether they may possibly 

 stand in a more intimate relation of affinity to an)' one subdivision of the group, or whether 

 they exhibit a similar relation to all. According to the tabular sketch already given, the 

 group of Aspidostraca is divided into three tribes, bearing the names of LojjJiyropoda, 

 PhllUopoda, and Pcccilopoda. These denominations indicate important diiferences with regard 

 to the feet ; for the first two divisions possess soft, membranous organs of locomotion, 

 solely calculated for swimming ; while the Fcecilopoda possess hard, articulated, walking 

 feet, and in this group serve also as gills, and aerate the blood. This circumstance has 

 already been taken into consideration, and an important difference between the animals of 

 this group and the Trilobites has been thence inferred. The genus Limidtits, which con- 



