Cbap. X.] aEOUPS OP ALLIED SPECIES. 81 



was a Chthamalus, a very common, large, and ubiqui- 

 tous genus, of which not one species has as yet been 

 found even in any tertiary stratum. Still more re- 

 cently, a Pyrgoma, a member of a distinct sub-family of 

 sessile cirripedes, has been discovered by Mr. Wood- 

 ward in the upper chalk; so that we now have abun- 

 dant evidence of the existence of this group of animals 

 during the secondary period. 



The case most frequently insisted on by palaeonto- 

 logists of the apparently sudden appearance of a whole 

 group of species, is that of the teleostean fishes, low 

 down, according to Agassiz, in the Chalk period. This 

 group includes the large majority of existing species. 

 But certain Jurassic and Triassic forms are now com- 

 monly admitted to be teleostean; and even some palaeo- 

 zoic forms have thus been classed by one high authority. 

 If the teleosteans had really appeared suddenly in the 

 northern hemisphere at the commencement of the chalk 

 formation the fact would have been highly remarkable; 

 but it would not have formed an insuperable difficulty, 

 unless it could likewise have been shown that at the 

 same period the species were suddenly and simultane- 

 ously developed in other quarters of the world. It 

 is almost superfluous to remark that hardly any fossil- 

 fish are known from south of the equator; and by run- 

 ning through Pietet's Palaeontology it will be seen that 

 very few species are known from several formations 

 in Europe. Some few families of fish now have a con- 

 fined range; the teleostean fishes might formerly have 

 had a similarly confined range, and after having been 

 largely developed in some one sea, have spread widely. 

 Nor have we any right to suppose that the seas of the 

 world have always been so freely open from south to 



