182 H. SPENCER HARRISON. 



is shed^ a,nd that successional teeth arise at different times to 

 fill their places. 



The number of teeth in the maxilla at this stage is eleven, 

 of which the last two are behind the otliers in development, 

 and are attached to a posterior free prolongation of the 

 dental lamina. The labio-dental and the internal maxillary 

 grooves with their glands are now obvious structures, and 

 the dental lamina arising from the deepest part of the 

 internal groove has a posterior prolongation, to which the 

 last tooth is still attached. The palatine has six teeth, the 

 last two resembling the corresponding maxillary teeth in 

 their state of development. 



The fifteen teeth of the mandible are succeeded poste- 

 riorly by two enamel organs, the posterior of which is in a 

 very early stage of development, the anterior being well 

 calcified. 



My observations had reached this stage when, through the 

 further kindness of Professor Howes, I was enabled to 

 examine two fine young specimens, measuring respectively 

 17*8 cm. and 21-2 cm. in total length. These, which filled 

 the gaps in the Howes- Swinnerton series, were by a fortu- 

 nate coincidence received at the very moment I was in need 

 of them. They were the gifts of Professor A. W. P. Thomas, 

 of Auckland, New Zealand, and from their study I am able 

 to show that there is not only a development of successional 

 teeth in Hatteria, as was first stated by Baur, but also an 

 actual tooth replacement, denied by him and others. We 

 may consider first the smaller specimen, 17'8 cm. total 

 length, 7"2 cm. from anterior border of pelvis to snout. 



In place of the three premaxillary teeth on each side, 

 as seen in Stage T (figs. 17 and 20), we find in this specimen 

 only two. It is clear that at any rate the second tooth 

 (counting from the middle line) has been shed. Not only so, 

 but the first has also fallen out, and the successional tooth 

 (figs. 18 and 20, t. s. 1.) has replaced them both, as seen in 

 an entire preparation of the right premaxilla (fig. 21). In 

 the same preparation (and figure) we see that No. 3 is 



