164 H. SPRNCER HABUTSON. 



presumed absence lie attributes the early wearing down of 

 the teeth. He found no true cement ; but bone corpuscles 

 and dentine tubules occur side by side, so that at this point 

 he considers we have to do with osteodentine. 



In 1899, in his paper on the '' Outlines of the Development 

 of Hatteria," Dendy (9) first described the presence of three 

 separate teeth in each premaxilla and on each side of the 

 mandibular symphysis, at a period just before hatching. He 

 suggests that fusion takes place later, the result being the 

 formation of the four anterior teeth of the adult. 



Howes and Swinnerton (10), in a paper at present in the 

 press, point out that the middle premaxillaiy tooth on each 

 side ceases to grow after a certain stage (Stage S), and is 

 in all probability shed. They also figure the replacing tooth 

 lying lingually to it, but owing to lack of material they 

 do not venture to decide whether this does or does not be- 

 come functional. They discuss Baur's observations, and 

 draw attention to the alternation in size of the teeth in the 

 embryos and young. They found no calcified vomerine 

 teeth in any of the embryonic stages investigated. As 

 regards the substance on the exposed surfaces of the denti- 

 gerous bones, they accept Tomes's (11) view that it consists 

 of true bone, and they have therefore introduced a new term 

 — ^^hyperacrodont" — for describing the secondarily theco- 

 dont condition produced in some of the teeth by the dis- 

 position of this bone round their bases. 



Material and Observations. 



I have examined complete series of sections taken in 

 various planes, through embryos and young of several stages, 

 ranging from about the third month of incubation (Q) to a 

 period some months after hatching. I have also examined 

 preparations, dissections, and specimens of young and adult 

 animals. In describing the stages I sliall make use of letters 

 and numbers in the same sense as Dendy, and as Howes 

 and Swinnerton, adding such other data as may be desirable. 



