256 Prof. Hughes, On the relation of the appearance [May 5, 



New forms or striking modifications of old forms appear in the 

 Wenlock which succeeds the May Hill Beds quite conformably, 

 and again in the Ludlow, which passes so gradually into the 

 Wenlock that in the absence of the Wenlock Limestone it is 

 almost impossible to draw a line between them. When we get 

 into the Limestone the immense abundance of corals, encrinites 

 and cystideans, as before pointed out, though they swell the 

 number of species, do not form fair ground of comparison with 

 non-calcareous strata. Many new forms of Trilobites appear, 

 Sphaerexochus, Acidaspis, Cyphaspis, &c., and whatever they be, 

 there is no suggestion of any unconformity in this series, even 

 carrying it over the great Lamellibranch zone at the top of the 

 Ludlow, and far above the Silurian through the Ledbury shales 

 into the Lower Old Red. 



Let us notice what becomes of the Trilobites eventually. 

 Several old genera, as Bronteus, Homalonotus, Phacops, Proetus, 

 get into the Devonian, tiding over what is locally at any rate a 

 great unconformity, and that is the last seen of those genera. All 

 the family of Trilobites die out long before the close of the Car- 

 boniferous, in which Phillipsia and Griffithides represent that 

 abundant family to which we had principally to refer in classify- 

 ing the Cambrian and Silurian Rocks. 



It is true that where the Trilobites die out the Limuli, repre- 

 sented long before by Neolimulus falcatus of the Wenlock, become 

 more common, but there is a great gap between these two groups ; 

 and there was not in the last Trilobites any approach to the Limu- 

 loid type. 



It seems that the evidence so far, making allowance for the 

 imperfection of the record and the limited search which has been 

 made in many areas, goes to show that whether we consider the 

 smaller groups, as varieties and species, or the larger as genera, 

 new forms appear at various horizons in uninterrupted deposits, 

 and that they die out in the same way; and that after a long 

 lapse of time, as measured by deposition, there is caeteris paribus as 

 great a change in the life of the period as we find after a similar 

 interval measured by denudation. 



We speak of higher and lower forms of life. . It is not meant 

 that the higher is better fitted for its surroundings than the lower, 

 but the term higher is applied to those forms which have a more 

 complex arrangement of organs for discharging the varied func- 

 tions of life. And there certainly seems to have been an increase 

 of higher forms as time went on. So it is interesting to take note 

 of the first appearance of some of these and test its bearing upon 

 the question We are considering. In the Lower Ludlow, i.e. high 

 up in the Silurian Rocks, we have the earliest yet known remains 

 of fishes. Yet we ought to have found them had they been there, 



