chap. IX.] TEETH : RHIXOPTERA. 259 



Selachii. 



Some features characteristic of Meristic Variation are well 

 seen in the case of the teeth of Sharks and Rays. Of these fishes 

 there are many having little differentiation between the separate 

 rows of teeth. In these a distinct identity cannot be attributed 

 to the several rows, and numerical Variation is quite common. 

 But besides these there are a few forms whose teeth are differ- 

 entiated sufficiently to permit a recognition of particular rows 

 of teeth in different specimens, and to justify the application of 

 the term "homologous" to such rows. Nevertheless with such 

 differentiation Meristic Variation does not cease. 



In the following examples it will be seen further that in such 

 Variation there may be not merely a simple division of single 

 teeth but rather a recasting of the whole series, or at least of that 

 part of it which presents the Variation, for the lines of division 

 in the type may correspond with the centres of teeth in the 

 variety. 



These cases also exemplify the fact that variations of some 

 kinds are often only to be detected when in some degree im- 

 perfect ; for if the divisions in No. 396 for instance had taken 

 place similarly on both sides, it would have been difficult to 

 recognize that this was a case of Variation. 

 1 9(5 > Rhinoptera jussieui (= javanica) : specimen in which the 

 number and arrangement of the rows of teeth is different on 

 the two sides, as shewn in Fig. 69, upper diagram. The dis- 

 position on the right side of the figure is normal, that on the 

 left being unlike that of any known form. Specimen in B. M. 

 described by Smith Woodward, Ann, and Mag. N.H., Ser. 6, 

 vol. i. 1888, p. 281, fig. As Woodward points out, the rows of 

 plates on the left side may be conceived as having arisen by 

 division partly- of the plates of the central row and partly from 

 the lateral row, marked I. But if this be accepted as a repre- 

 sentation of the relation of the normal to the abnormal, in the 

 Avay indicated by the lettering, the plates of the row marked b, 

 for instance, must be supposed each to belong half to one rank 

 and half to a lower rank. The same applies to the plates in the 

 row I b. By whatever cause therefore the points of develop- 

 ment of the teeth are. determined, it is clear that the centres 

 from which each of the teeth in the rows I b and b was de- 

 veloped were not merely divided out from centres in the normal 

 places but have undergone a rearrangement also. With change of 

 number there is also change of pattern. 



The tessellation on the abnormal side is so regular and definite 

 that had it existed in the same form on both sides the specimen 

 might readily have become the type of a new species. 



There is indeed in the British Museum a unique pair of jaws 

 in both of which (upper and lower) a very similar tessellation 



17—2 



