44 CHARLES S. TOMES. 



in its cell now becomes attached to the anterior part of the 

 cell, and the complicated system of the special retractor and 

 protractor muscle, all the mutations have not yet taken place 

 in the Rhabdopleura. This animal lias thus remained sta- 

 tionary in the first stage of development from a Hydrozoon 

 to a Polyzoon, but must, nevertheless, be considered as be- 

 longing to the type (trunk) of the Polyzoa, since the 

 development of the completely organised digestive system, 

 which is so entirely foreign to the Hydrozoa, sufficiently 

 stamps it as a Polyzoon, 



Finally, we remark that it may appear strange that the 

 Rhabdopleura, which in all probability is of so ancient origin, 

 should possess a similar, although modified, form of tenta- 

 cular Corona (bilateral) to that which belongs to most fresh 

 water Polyzoon (P. Hippocrepia, Gervais, Phylactolaemata, 

 AUman), and which is usually considered as a more perfect 

 formation that the circular (in F. infundibulata, G.) . It is, 

 however, possible, that the first is properly the circular 

 form^ from which the latter has subsequently arisen. The 

 fresh waters appear, as Hseckel lately has remarked, to con- 

 tain the direct descendants of some of the eldest animal 

 forms which, by reason of the less complicated accident of 

 the fresh waters, have often in the " struggle for life,^'' only 

 slightly altered their original more simple structure ; as for 

 instance, among the Coelenterates, the Hydra ; among the 

 Rhizopods, the Aclinophrys, Gromia, and the shell-less 

 Radiolaria lately discovered by Focke ; among the fish, the 

 Ganoidea, &c. 



On the Existence of ati Enamel Organ i/i an Armadillo 

 {Tatusia Peba). By Charles S. Tomes, M. A. (With 

 Plate II.) 



According to the views of Arnold and Goodsir, which for 

 many years have passed current amongst anatomists as being 

 a correct interpretation of the early stages of tooth develop- 

 ment, the dental germs first make their appearance as free 

 uncovered papilla3, rising up from the bottom of an open 

 groove, to Avhich was applied the name " primitive dental 

 groove ; " and our anatomical text-books have hardly kept 

 pace with advancing knowledge in this matter, for even in 

 some of the most recent these views of Arnold and Goodsir, 

 now in some important particulars obsolete^ are to be found, 



