262 



MERISTIC VARIATION. 



[part I. 



divided, but the division is gradually lost towards the middle of the 

 jaw and the external plates are without trace of division. C. S. M. 



Fig. 71. The lower jaw of a Selachian, No. 400. The proximal ends shewn 

 (enlarged). The right is reversed for comparison with the left. 



Eadul^e of a Gasteropod. 



The following example of Meristic Variation in the teeth 

 of a Molluscan odontophore may be taken in connexion with 

 the subject of teeth, though the structures are of course wholly 

 different in nature. For information on this subject I am ^in- 

 debted to the Rev. A. H. Cooke. 



Generally speaking the number and shapes of the radular 

 teeth are very characteristic of the different classificatory divi- 

 sions. There are however certain forms in which a wide range 

 of Variation is met with ; of these the case of Buccinum itndatum is 

 the most conspicuous. 

 *401. Buccinum undatum. In most specimens the number of 

 denticles on the central plate is 5 — 7 and on the laterals 3 — 4. 



In 27 specimens from Hammerfest and Vardo the teeth were 

 as follows : — 



Lateral plates. Cases. 



4 ' 8 



4 12 



4 2 



4 1 



4 1 



3&4 1 



3& 4 1 



4 & 5 1 



Central plate. 

 5 

 6 

 7 

 6—8 

 9 

 6 

 7 

 8 



from Friele, Jahrb. dent. mal. Ges., vi. 1879, p. 257. 



