A Suggestion on Extinction. 467 



species, is very remarkable. Lagena, Nodosaria, and Textu- 

 laria are in each case simple forms in the groups in which 

 they occur, and with great uniformity of architecture. The 

 two former range from the Silurian, and the latter from the 

 Carboniferous to the present day. On the other hand, we 

 find very specialised forms often of large size, such as 

 Fusulina and Schwagerina, with a world-wide distribution, 

 during the Carboniferous, where they occur in such myriads 

 as to form great beds of limestone. They are, however, 

 entirely confined to that system. The development of 

 enormous species of Nummulina in the early Tertiaries, 

 with a complicated canal system, took place with remark- 

 able rapidity, and over a wide area. This development, 

 again, however, was exceedingly short lived. The Graptolites, 

 which had a world- wide distribution in early Palaeozoic times, 

 must have existed in enormous profusion, and underwent a 

 degree of variation which is very extraordinary considering 

 the simplicity of their type of architecture. They, however, 

 suddenly disappeared at the end of Silurian times. 



The genus Bictijonema, which, if a Graptolite at all, has a 

 very generalised organisation, ranges from the Cambrian to 

 the Devonian. The two genera, Diplograptus and Climaco- 

 graptus, which appear to be the more generalised genera of 

 the true Graptolites, occur throughout the Ordovician and 

 Silurian rocks, a peculiarly long range compared with other 

 genera, which appear to be more specialised, and have much 

 more restricted ranges. 



The Lithistid and Hexactinellid sponges have existed since 

 early Cambrian times. The Calcispongise, however, are not 

 found till late in the Palaeozoic, and only became of any 

 importance in the Mesozoic period. Of these latter, the 

 Fharetrones, which have the largest and most complex 

 organisation, underwent complete extinction, following their 

 greatest development in the Cretaceous. The great variety 

 in the vegetative architecture in the genera of fossil sponges 

 is only as remarkable as the very short range in time shown 

 by the more extraordinary forms throughout geological his- 

 tory. It might be suggested that the possibilities of cross- 

 fertilisation amongst forms like the sponges, which depend 



