i26 SCIENCE PROGRESS 



family seems to be moribund, a circumstance not entirely due 

 to human agency. 



Turning to the early forerunners of fishes, we find that 

 the heavily armoured Ostracoderms, which flourished in the 

 Silurian and Devon, did not survive the latter period ; their 

 part in the economy of nature was thenceforward represented 

 by the more highly organised true fishes of the Ganoid type. 

 In this group a marked progression is traceable in the abandon- 

 ment of armour and the evolution of more and more flexible 

 types, through the Amioidca to the modern Teleostei, which show 

 such remarkable variability and adaptability to all conceivable 

 circumstances. 



Among the Mollusca several striking examples of the advan- 

 tages resulting from a reduction of armour are to be found both 

 among the Cephalopoda and the Gastropoda. The massive 

 guard of the Bclemnitidce, so successful a group in the Mesozoic 

 era, became reduced in size in course of time and finally dis- 

 appeared ; some of the descendants of the Bclemnitidce continued 

 to retain the phragmocone, either well developed but modified 

 into a spiral shell as in Spirula or else flattened out and 

 accompanied by a very rudimentary guard as in Sepia. Other 

 members of the group, on the other hand, retained and developed 

 still further the horny pen or pro-ostracum of the belemnite, as 

 in the Calamaries. In this group the calcareous secretion of 

 shell-substance has been entirely abandoned and it is here that 

 we find the Cephalopoda reaching their highest development, 

 often attaining the gigantic length of 40-60 ft., 1 as in Architeuthis 

 and its allies, which probably form the basis of most of the 

 sea-serpent stories. 2 Finally, the octopus, which in the adult 

 stage has lost all trace of a shell (excepting, according to Owen, 

 for two short, cartilaginous stylets embedded in the dorsal 

 mantle), shows indications of a shell only during its develop- 

 ment. 



In Gastropoda there are several independent lines along 

 which a reduction of the shell has taken place, accompanied 

 by a greater predominance and a wider distribution of the race, 

 by a frequent increase in size and usually by the adoption of 

 carnivorous habits. Among the Euthyneura, the Pulmonata 



1 Verrill, Amer.Journ. of Scie?ice and Arts, 1875, > x - PP- I2 3» l 77- 



2 Lee, H., Sea Monsters Unmasked, London, 1883; and Hoyle, W. E., Proc. 

 Roy. Phys. Soc. Edinburgh, ix. 



