42 THE AGE OF THE EARTH 



described in the genera Conularia, Hyolithes, Pterotheca, 

 &c., are true Pteropods, as they are supposed to be, 

 they occur in the Cambrian and Silurian strata, while 

 the group of Gastropods from which they almost certainly 

 arose, the Bullidae, are not known before the Trias. 

 Furthermore, the forms which are clearly the oldest 

 of the Pteropods Limacina and Spirialis are not 

 known before the beginning of the Tertiary Period. 

 Either there is a mistake in the identification of the 

 Palaeozoic fossils at Pteropods, or the record is even 

 more incomplete than usual, and the most specialized 

 of all Molluscan groups had been formed before the 

 date of the earliest fossiliferous rocks. Even if this 

 should hereafter be disproved, there can be no doubt 

 about the early appearance of the Molluscan Classes, 

 and that it is the irony of an incomplete record which 

 places the Cephalopods and Gastropods in the Cambrian 

 and the far more ancestral Chiton no lower than the 

 Silurian. Throughout the fossiliferous series the older 

 families of Gastropods and Lamellibranchs are followed 

 by numerous other families, which were doubtless derived 

 from them ; new and higher groups of Cephalopods were 

 developed, and, with the older groups, either persisted 

 until the present time or became extinct. But in all 

 this splitting up of the Classes into groups of not widely 

 different morphological value, there is very little pro- 

 gressive modification, and, taking such changes in such 

 a period as our unit for the determination of the time 

 which was necessary for the origin of the Classes from 

 a form like Chiton, we are led to the same conclusion 

 as that which followed from the consideration of the 

 Appendiculata, viz. that the fossiliferous series would 

 have to be multiplied several times in order to pro- 

 vide it. 



Of the Phylum Gephyrea, I will only mention the 

 Brachiopods, which are found in immense profusion in 

 the early Palaeozoic rocks and have occupied the sub- 

 sequent time in becoming less dominant and important. 

 So far from helping us to clear up the mystery which 

 surrounds the origin of the Class, the earliest forms are 



