474 Proceedings of the Royal Physical Society. 



mum of numbers of genera and species in the Ordoviciau, 

 after which they slowly declined till their complete extinc- 

 tion in the Permian. The accompanying Chart, based upon 

 Zittel, shows the ranges of the different families, with the 

 number of genera and subgenera. It is to be noticed that a 

 small number of genera in a family tends to be associated 

 with a long range in time, and that these genera, on the 

 whole, show longer ranges than those of the more variable 

 families. A list of British Trilobites, with the longest ranges, 

 shows that they also fall within these families. 



Limulus, the living representative of the Merostomata, is 

 found in the fossil state as far back as the Trias. These 

 fossils show that it has undergone very little change, and 

 the forms now living in widely separate seas are very similar. 

 It would appear, therefore, to be a very stable type. Limulus 

 is another of those forms which show a great resistance to 

 death. 



Of the Palaeozoic Eurypterids, by far the longest range is 

 attained in Eurypterus, Silurian to Permian. The- gigantic 

 Pterygotus, which seems a more specialised genus, is only 

 found in the uppermost Silurian and Old Eed Sandstone. 



The Scorpions, which appear to be related forms, existed 

 along with these gigantic Merostomata, and range from the 

 Silurian to the present day with remarkably little change. 



It is, however, in the study of the extinct Vertebrata that 

 it seems we are likely to arrive at any conclusion as to the 

 results of rapid specialisation. Unfortunately, my knowledge 

 is necessarily very limited, and I will only attempt to point 

 out a few of the most general points bearing on the question. 



Amongst the fishes, it is easily observed that the extinct 

 groups, like the Ostracodermi and the Arthrodira, attained a 

 very high degree of specialisation in their own direction 

 before they became extinct. On the other hand, the 

 Elasmobranchs, which have had such an extraordinary 

 persistence, appear to have retained in laige measure a very 

 primitive degree of organisation, although so specialised in 

 other directions, and having acquired a method of reproduc- 

 tion allied to the higher vertebrates. 



Whether the existing Dipnoi and Crossopterygian fishes, 



