46 CHARLES S. TOMES. 



In the order Opliidia an interesting and eminently charac- 

 teristic arrangement is met with. A very large number of 

 successional tooth-germs are always in course of preparation ; 

 thus there are usually about eight or nine successional poison 

 fangs in various stages, and six or seven sacs at the base of 

 the ordinary teeth (see e-^, €2, e^ in fig. 4). 



These successional sacs are arranged together in a sort of 

 nest, within a connective-tissue capsule, an arrangement 

 likely to prove advantageous when the mouth is dilated to 

 swallow prey ,• each one consist of an enamel organ, a 

 dentine pulp, and a feeble connective-tissue investment. 



The youngest tooth-germ is to be found at the bottom of 

 the series {e^ in fig. 4) ; it originates from a budding down 

 of an ' enamel germ " from the neck of the enamel organ of 

 the tooth-germ next above it, and the subsequent or coinci- 

 dent formation of a dentine papilla (see i in figure) ; not only- 

 do the tooth-germs of all Ophidia possess enamel organs, but 

 these actually form a thin coating of enamel ; and Professor 

 Owen was mistaken in supposing that enamel was absent, 

 but that the teeth of " all Ophidia consist of dentine and 

 cement." No Ophidian tooth, so far as the examination of 

 a very large number of specimens can decide, possesses 

 cementum at all. 



In Batrachia the process is in all essentials similar, though 

 it is subject to modification according to position. In the 

 newt the chain of successional tooth- sacs is very beautifully 

 seen (see fig. 6), an enamel-germ, Avhich as yet has no cor- 

 responding dentine-pulp, extending beyond the youngest of 

 the series (c in fig. 6). 



The tooth-sacs of the newt have the peculiarity, first noted 

 by Dr. Lionel Beale, that they have no capsular investment 

 whatever. They consist solely of aggregations of cells, and 

 under pressure readily break up ; the enamel organs form 

 a spear-like point of enamel upon the teeth, but the enamel 

 does not extend down upon the sides of the teeth. 



In the frog the edentulous lower jaw closes within the 

 upper teeth, and so little space is left that an arrangement 

 like that seen in the newt becomes imjjossible. In fig. 5 an 

 enamel germ is seen at the inner side of the base of the 

 tooth in place ; as the new tooth-sac attains to any consider- 

 able size, space is made for it by the absorption of the base 

 of the older tooth, into the pulp cavity of which it finally 

 passes, recalling in some measure the manner of succession met 

 with in the crocodiles. It is also a question whether the new 

 enamel germs really do spring from the necks of the enamel 

 organs of older tooth-sacs, or whether they originate de novo 



