496 SCIENCE PROGRESS. 



animals which conceal themselves in the ground at the sea- 

 bottom, and probably use their snouts in hunting for prey, 

 although I am not acquainted with any particular observa- 

 tions on the performance of such actions by them. On the 

 the other hand it may be objected that a large number of 

 Selachians are as pelagic and active in their habits as any 

 fishes, never use their snout for probing the ground, but 

 pursue other active fishes in mid-water. This is true of the 

 common spiny dog-fish, Acaiithias vulgaris, and of the large 

 species of Carcharias living in tropical seas. I cannot pre- 

 tend to be able to explain all the difficulties presented by 

 such species, but it is certainly a fact that there are among 

 them differences in the degree of prolongation of the snout, 

 and that when the mouth is nearer to the end of the head, 

 the tail is more bent up so as to approach the homocercal 

 condition. This correlation is evident enough in Lt^mar- 

 gus boreal IS, the Greenland shark, and in Selache iuaxima, 

 the basking shark. Concerning the aperticolous habits of 

 the latter there can be no doubt, since it lives on plankton, 

 while the Greenland shark has a great avidity for whale's 

 flesh, and also devours fishes. 



It is a remarkable fact that the most ancient Selachian 

 forms of which anything like complete remains have been 

 preserved do not resemble the recent torms in the characters 

 of snout and tail. The mouth in them is terminal, and the 

 tail either diphycercal or nearly homocercal. In Pleura- 

 ca7tthus we find the former condition ; in Acantliodes, the 

 ventral lobe of the caudal fin is well developed while the 

 upturned caudal extremity of the body is short, so that we 

 have an approach to a secondary symmetry which in 

 Cladoselache of the Lower Carboniferous becomes remark- 

 ably perfect, the dorsal and ventral lobes being equal, 

 although the dorsal is still formed not by fin-rays but by 

 the upturned extremity of the trunk. 



Such facts as these strongly suggest that it is possible 

 to over-estimate the importance of chronological position 

 in interpreting the evolution of animal forms. Certain 

 adaptations are found in unexpected perfection in geolo- 

 gical periods which have been supposed to be characterised 



