2 F. R. Cotvper Heed — Some Wenlock Species of Lichas. 



opportunity of pointing out another possible cause of extinction of 

 some groups of animals. It will be observed that in many cases the 

 evolution of a group of animals is accompanied by a simultaneous 

 increase in the bulk of the individuals composing it. The Proboscidea 

 themselves offer a fairly good instance of this tendency, but a better 

 knovpn case is that of the horses. An almost necessary corollary of 

 this increase in bulk is the lengthening of the individual life, or at 

 least what, for our argument, amounts to much the same thing, the 

 lengthening of the time taken to attain sexual maturity. In many 

 Ungulates this increased longevity is indicated by various modi- 

 fications of the teeth, tending to give them a longer period of wear : 

 generally this end is attained by the increasing hypselodonty of the 

 cheek-teeth. A necessary consequence of the longer individual life 

 will be that in a given period fewer generations will succeed one 

 another, and the rate of evolution of the stock will therefore be 

 lowered in the same proportion. If now the conditions of life 

 undergo change, the question whether a given group of animals will 

 survive or become extinct will depend upon whether it can undergo 

 sufficiently rapid variation to enable it to avoid getting so far out of 

 harmony with its surroundings that further existence becomes 

 impossible. It seems to follow then that the smaller animals, in 

 which the generations succeed one another rapidly, will have a better 

 chance of surviving than the la'rger and more slowly breeding forms, 

 which at the same time will be still further handicapped if, as is 

 usually the case, they are more highly specialized than the smaller 

 forms, and therefore have a more restricted range of possible 

 variation. 



Another result of the increased length of the individual life would 

 be that, during the earlier history of a stock the modification under- 

 gone by its members would be more rapid than among the later forms, 

 a phenomenon of which actual instances might be cited, e.g. in the 

 Proboscidea. On the same principle it may perhaps also be explained 

 why certain groups have remained comparatively unchanged through 

 long periods of time. The Sirenia may be a case in point, though 

 no doubt in this case, among other factors, the comparative stability 

 of the conditions of life has had much to do with the conservative 

 character of the group. 



II. WOODWARDIAN MuSEUM NoTES : On SOME WbNLOOK SpBCIES 



OF Lichas. 



By F. R. CowPEK Eebd, M.A., F.G.S. 



(PLATE I.) 



rnHE entire collection of Wenlock fossils made by T. W. Fletcher, 

 X Esq., which is now in the Woodwardian Museum, affords 

 unsurpassed facilities for studying the material on which a large 

 number of species of trilobites wei'e founded. Particularly is this 

 the case with those of the genus Lichas, and a recent examination 

 of the types and other specimens which Fletcher used in writing 

 his paper " Observations on Dudley Trilobites " (Q. J.G.S., 1850, 



