386 NATURAL SCIENCE. June, 



developed teeth. In the skates (e.g., Raia), and some dogfish (e.g., 

 Scyllium), and sharks (e.g., Odontaspis), several rows of teeth are in use 

 at a time ; but in many sharks (e.g., Carcharias, Galeocevdo), where the 

 edge of the jaw is thinner, there is but a single row of functional 

 teeth in each jaw. In the latter forms it is unusual to find adjacent 

 teeth shed simultaneously. The replacement takes place alternately, 

 so that there is never a large vacuity left in functional dentition. 

 This provision against leaving any large extent of jaw unarmed is 

 identical with that which is to be observed, with even more regular 

 alternation, in the python, crocodile, and many other reptiles. In the 

 jaw of the Greenland shark (Lcvmargus) the teeth overlap one another 

 and interlock so closely that alternate succession is impossible. The 

 successional teeth, therefore, rise together en bloc, and stand slightly 

 higher than the older teeth. Seeing that the latter are not shed until 

 some time after replacement, the dentition never for a moment loses 

 its effectiveness. 



The teeth of Holocephali and Dipnoi persist for life and grow 

 continuously. These teeth are considered by Rose (7) to have 

 originated by a fusion of separate tubular denticles, a view which is 

 borne out by palaeontology ; but he unfortunately compares the teeth 

 of Dipnoi with those of Myliobatis, the tubular structure of which is 

 due to a sub-division of the pulp of the individual teeth (see p. 381). 

 The horny teeth of Cyclostomata are vertically replaced. In the 

 common lamprey, Petvomyzon, as many as three hollow cones may be 

 found superimposed, separated from one another by a soft white tissue, 

 which, like the cones themselves, is epidermal in origin, and which is 

 in marked contrast with the dark yellow or brown colour of the horny 

 material of the teeth. Of these three cones, the external one 

 (Fig. 17, t) is the tooth in use, and the other two (f and t") are 

 replacing teeth. A reserve tooth is thus ready for use immediately 

 its predecessor is shed. 



In most bony fishes, where the teeth are numerous and closely 

 packed, the succession is irregular (e.g., cod). In those, however, 

 where the teeth are less numerous, an alternate replacement, calcu- 

 lated to gain the same advantages as in Carcharias, may frequently be 

 found (e.g., Thyvsites). The succession of socketed teeth is usually 

 vertical. The young tooth is formed at the side of the older one, but, 

 by absorption of the base of the latter, comes to lie vertically below it, 

 and ultimately occupies the same socket (e.g., Spliyrcena, Chrysophrys). 

 A most perfect instance of vertical succession is seen in the pharyngeal 

 teeth of the wrasse (Fig. 20). In the extinct lepidosteoid Lepidotns, 

 the replacing tooth undergoes a complete inversion before it finally 

 comes into position (5 and 6). When first formed it has its pulp- 

 cavity directed towards that of the tooth which it will replace, while 

 its crown points in the opposite direction. The tooth then gradually 

 turns over (Fig. 13, t'), and by the time it supplants the old tooth it 

 has turned through an angle of 180 degrees. 



