LIFE-FOEMS OF THE PAST AND PKESENT. 
397 
being the most satisfactory explanation by which to account 
for their longevity. 
The group which has attracted perhaps the greatest amount 
of attention, and upon which vast labour has been bestowed 
with little or no result commensurate therewith, is that of the 
Trilobites. 
This is a truly palaeozoic group, and, so far as we are aware, 
an extinct group, although this is always a difficult point to be 
dogmatic upon. 
Look, for example, at the Limulidae. We have living King- 
crabs as far apart as on the east coast of North America and 
along the coasts of China and Japan, &c. We find them again 
in the Oolitic beds of Solenhofen in Bavaria. Again in the 
Coal-measures of England and Illinois; again in the Upper 
Silurian of Scotland. 
We cannot doubt the continuity, but the gaps are enormous. 
Nor can we, by the same rule, positively assert that the 
Trilobita are a strictly palaeozoic type, and will never be found 
in neozoic strata. 
In taking a general review of this great family or order, so 
widely distributed through the older sedimentary deposits, one 
is naturally struck by the immense amount of variation of 
form , brought about simply by the modification of a single 
plastic type, and that, apparently, a very elementary one. 
Yet by means of diverse ornamentation, in the way of spines, 
warts, and tubercles, by compression in one direction, by 
elongation in another, by adding to the normal number of 
segments of the body, or by subtracting therefrom ; by en- 
largement or reduction of the eyes (the only organs seen upon 
the dorsal aspect of the body), we obtain — as with one of those 
amusing human faces in vulcanite india-rubber that children 
delight to contort — all those endless modifications of expression 
of form possible to the frame of the highest vertebrate organism. 
In the present state of our knowledge it is very difficult to 
speak at all positively of the organisation of this group, but I 
incline to the belief that they conceal beneath their apparently 
simple structures evidence of more than one order. 
For instance: they indicate, in some points, a close relation- 
ship to the Phyllopoda. We find in both the articulated 
labrum ; and although we have not as yet obtained clear evi- 
dence of the maxillae in the Trilobita, we are justified in con- 
cluding from analogy that they possessed such organs.* Again, 
* See note on the “ Palpus and other Appendages of a Trilobite,” by H. 
Woodward j and a paper on the “ Supposed Legs of Trilobites/’ by E. Bil- 
lings ; “ Quart. Journ. Geol. Soc.” 1870, Vol. XXVI. See also H. Wood- 
ward “On the Structure of Trilobites,” “Geol. Mag.” 1871, Vol. VIII. p. 
289, Pl. VIII. 
