140 J. T. WILSON AND J. P. HIT.L. 



a very early stage of development, and corresponding with 

 the one he actually figures in the lower jaw. 



From an investigation of the dentitional condition of a 

 foetal Ornithorhynchus of a stage closely resembliug that 

 examined by Mr. Poulton, we are enabled, as we believe, to 

 throw some additional light on the subject under consideration. 



This specimen (designated as " Beta " in our collection) 

 measured 250 mm. along the dorsal curvature of the body 

 from the tip of the snout to the tip of the tail, and 107 mm. 

 measured in a straight line from the vertex of the head to 

 the dorsal convexity of the tail. This latter measurement is, 

 however, practically worthless for comparison, since it depends 

 on the acuteness of curvature of the specimen, and this is by 

 no means constant. Poulton's specimen was stated to be 

 83 mm. long " in the curled-up attitude in which it had been 

 received," fixed by the alcohol. From the published drawing 

 of the specimen (3, PI. XV a) "we estimate its dorsal contour 

 line from tip of snout to tip of tail to have been practically 

 the same as our own, viz. 250 mm. 



Our specimen was in excellent condition. Its head was 

 removed and divided into two portions slightly to one side 

 of the median plane by an incision which was not quite 

 parallel to the latter. The major (left-hand) portion was 

 imbedded in celloidin and decalcified in nitric alcohol. The 

 smaller (right-hand) portion Avas also decalcified and imbedded 

 in paraffin. Serial transverse sections from both sides of the 

 head were thus available for examination. 



In addition to this specimen " Beta" we have also investi- 

 gated the dentitional condition of a younger "mammary 

 foetus," whose external characters were minutely described 

 and figured by one of us a number of years ago (9). The 

 dorsal contour-line of this specimen (catalogued as "Delta" 

 in our collection) measured 80 mm. from tip of snout to tip 

 of tail. Both sides of the head of this specimen have been 

 examined in comjoletc transverse sectional series. The 

 sections from one side are thick, and were cut in celloidin 

 and stained in picric-hajmatoxylin. Those from the other 



