DEVELOPMENT OF TEETH IN HATTERIA PUNCTATA. 173 



fiable^ since at a later stage a similar ingrowth is found 

 opposite each tooth in the upper jaAV, with the position and 

 appearance of a dental lamina^ though the result of its further 

 development is mainly a glandular groove. On the whole I 

 regard the alternation in size of the mandibular teeth as 

 indicative of the presence of teeth belonging to the two 

 dentitions, which I have shown to be probably represented in 

 the premaxillte, maxillte, and palatines. I shall have occasion 

 to refer to this point again in the sequel. 



Posteriorly the dental lamina (which is probably already 

 involved in the ingrowth mentioned above) leaves the epi- 

 dermis and runs back for some distance deep in the meso- 

 derm, until it terminates between the coronoid and dentary. 

 This portion of it is probably entirely dental in character, 

 and bears one enamel organ. 



In view of the complexity of the epidermal down-growths 

 into the mesoderm at this and following stages, it will be 

 conducive to clearness if a short account of the arrangement 

 of the parts of the mouth in the adult in the neighbourhood 

 of the teeth be given here. 



In both upper and lower jaws the lips and teeth are, as is 

 usual, separated by a deep groove, for which we may use the 

 term labio-dental groove. In the lower lip there is a sort of 

 glandular shelf running along the internal face, and the 

 groove itself has glands on the face looking towards the 

 teeth (figs. 14 and 19, /. d. gr.). The groove is developed 

 from the labio-dental strand (figs. 4 — 9, vi. lah., man. Icih.). 

 Running along the inner or lingual face of both jaws is 

 a prominent fold or ridge of mucous membrane, reaching 

 almost as high as the apices of the teeth. That of the lower 

 jaw is the crista gingivalis inferior (8), that of the 

 upper the crista gingivalis lateralis. The deep groove 

 between the crest and the mandible may be called the 

 mandibular (internal) dental fossa, and that in the corre- 

 sponding- position in the upper jaw the maxillary (internal) 

 dental fossn. In Hatteria these grooves are formed from 

 epidermal ingrowths, situated lingually to the teeth (and 



