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Memoir Sears Foundation for Marine Research 



level than the series that has replaced them. Consequently, the jaw, as viewed from the 

 outside, may show two series (perhaps even three or only one), while the series next 

 younger than those in actual use (one, or possibly two) may be either oblique or may still 

 lie in the reversed position (Fig. 7), depending on the momentary stage of replacement. 

 It is probable, also, that replacement of teeth is similar (i.e., by series rather than singly) 

 in the Smooth Dogfishes (Mustelus), in which they are arranged in mosaic.' 



Figure 7. Semidiagrammatic cross-sections of upper 

 jaws (left) and lower jaws (right) of two adult speci- 

 mens of Squalus acanthias illustrating difTerent stages 

 in the replacement of teeth. 



Form, Activity, Size. Most sharks are subcylindrical in form 5 some are as beautifully 

 streamlined as the larger members of the mackerel tribe, giving rise to the vernacular 

 name "Mackerel Sharks." On the other hand, a few are very much flattened dorso-ven- 

 trally and expanded laterally, so that they resemble skates or rays in general appearance. 

 There is a wide variation in their swimming also. The Mackerel Sharks (Isuridae) are 

 exceedingly active, swift and powerful, whereas others, such as the Greenland and Portu- 

 guese Sharks (Somniosus, Centroscymnus) are so sluggish and inert that it is a question of 

 some interest how they succeed in capturing their prey. Sharks also vary widely in size. 



5. Cawston, in a series of recent papers, has maintained that the reserve teeth in sharks come into use only when an 

 individual tooth, lying in front, happens to be lost through injury (Brit. dent. J., 55, 1938: 321 j S. Afr. J. Sci.i 

 3S> '938: 321 ; Dent. Rec, 59 [10], 1939: i ; Dent. Rec, 60 [11], 1940: 435; S. Afr. dent. ]., 14 [12], 1940: 

 312; ridskr. Wetensch. en KunS; 2, 1941 ; S. Afr. dent. J., ly, 1943: 117; S. Afr. dent. J., ly, 1943: 295; Copeia, 

 1944: 184). However, direct observations on the shedding of teeth in Carcharias (Breder, Copeia, 1942: 42) 

 have proved that the shedding of the older teeth, and their replacement by younger teeth, is a normal process. 

 Successive stages in the process, such as those illustrated in Fig. 7 for the Spiny Dogfish {Squalus acanthias), are 



