126 



Mr. 0. Thomas. 



[May 16, 



tlie nerve cord. The cells forming these ridges are continuous with 

 the peritoneal lining of the body wall, and in this region the 

 elsewhere flattened epithelium has become modified to form the 

 ova in the female and the mother-cells of the spermatozoa in 

 the male. The ridge is thickened at its free end, and here the 

 more mature generative cells are found. When ripe these dehisce 

 into the body cavity in which they live for some time and increase in 

 size. The ova found in the ccelomic fluid are oval in outline, their 

 nucleus is very large, and they are surrounded by a thick zona radiata. 

 No ripe spermatozoa are found in the coelom, but their mother-cells 

 exist in various stages of division. 



Conclusions. — The more important contributions to the anatomy 

 of the Grephyrea contained in the foregoing Abstract are the descrip- 

 tions of (i) the head with the extensible collar, (ii) the skeletal 

 structures, (iii) the minute anatomy of the nephridia, and (iv) the 

 histology of the nervous system and sense organs. 



The most important conclusions to be drawn from these facts seem, 

 in my opinion, to confirm those systematic writers who assign Phoronis 

 a position in the immediate neighbourhood of the unarmed Grephyrea. 

 In addition to the points upon which they lay stress in the general 

 anatomy of these forms, I would point to the close similarity in struc- 

 ture and position of the skeletal tissues in both groups, and the possible 

 homology between the collar in Phi/mosoma and the extensible calyx 

 which surrounds the head in Phormis. 



VIII. " On the Dentition of OrnithorJiynchus." By OLDFIELD 

 Thomas, Natural History Museum. Communicated by 

 Dr. Gunther, F.R.S. Received May 8, 1889. 



[Plate 2.] 



At the meeting of the 9th of February, 1888 * Mr. E. B. Poulton 

 communicated to this Society the first discovery of the presence of 

 teeth in Ornithorhynchus, a discovery which naturally awakened 

 extreme interest throughout the scientific world. This first account 

 was afterwards elaborated into a long and excellent description! of 

 the form, structure, and development of the teeth, and their relation- 

 ship to the horny plates (or " cornules," as they may be conveniently 

 termed) which form the functional masticatory organs of the adult 

 animal . 



Apart from the valuable histological descriptions, the conclusions 

 put forward in Mr. Poulton's paper may be briefly epitomised as 

 follows : — 



* < Roy. Soc. Proc.,' vol. 43, 1888, p. 353. 



f * Quart. Journ. Microsc. Sbi.,' vol. 29, 1888, p. 9. 



