III. 

 The Evolution of Sharks' Teeth. 



So much has lately been written concerning the development and 

 modes of complication of the teeth in the highest vertebrates, the 

 Mammalia, that it may be of interest to summarise our present know- 

 ledge of the phenomena in question, as illustrated by some of the 

 lowest vertebrates, the Elasmobranchs or Sharks and Skates. If the 

 teeth in mammals are frequently complicated and modified by the 

 fusion of adjacent cusps and by their splitting (as now claimed by 

 most competent observers), it is probable that the fundamental laws 

 governing such modifications will produce analogous results under 

 the simplest conditions as exemplified in the dentition of the cartila- 

 ginous fishes. In each case the known palaeontological history of the 

 various groups of animals is tolerably extensive ; and it is thus pos- 

 sible to institute satisfactory comparisons in all essential particulars. 



I. — The Fusion of Adjacent Cusps. 

 It is a familiar fact that the calcified tubercles in the skin of 

 numerous Elasmobranchs, both living and extinct, are often fused by 

 their base into little groups, though generally without much constancy 

 or regularity of arrangement. It is thus to be expected that the same 

 complication of the tubercles will occur in the mouth, where their func- 

 tion as teeth may be deemed to specially favour this process. The 

 fusion does indeed take place, and the only peculiar features to be 

 noticed are the constancy, regularity, and symmetrical arrangement 

 of the little groups of cusps or " teeth." In all the most primitive 

 sharks, the inserted base of the cuspidate teeth is a great flattened 

 expansion, exactly like that of each tubercle or group of tubercles in 

 the external skin ; and the polished shining cusps are at first always 

 well separated above the base. Such teeth are well-known from 

 Palaeozoic rocks under the names of Diplodus, Cladodus, &c., and it may 

 be remarked that they never consist of more than a single row of 

 points, thus showing that they originate from adjacent cusps 

 developed simultaneously in one and the same series. The relative 

 sizes of the cusps, of course, vary much in the different sharks, as also 

 do their numbers in the respective " teeth " ; and in some cases the 

 actual fusion of the cusps well above the base begins to occur, more 

 especially when they assume obtuse proportions. 



