RECORDS 45. 



narrowing of the anterior portion of the premaxillaries, reduc- 

 tion of all the anterior teeth, and by elongate horns placed im- 

 mediately over the eyes. In Brontotheriiuii, the horns are by 

 far the largest and most powerful, and acquire an extreme an- 

 terior position, absorbing the free portion of the nasals ; all the 

 upper cutting teeth persist ; great buccal plates are evolved ; 

 and the skull measured along the base line is extremely brachy- 

 cephalic. The four types were illustrated by models and 

 diagrams. 



Professor Bashford Dean considered briefly some points in 

 the development of sharks, and attempted to reduce the type 

 of the early development of the recent types to that of their 

 holoblastic ancestor. This form probably occurred within the 

 strict limits of the group Elasmobranchii — for the absence of 

 clasping organs in the palaeozoic genera of Acanthodians and 

 Cladoselachids predicates external fertilization, and eggs many 

 in number and of small size. In the line of this comparison, 

 reference was made to the early development of the Japanese 

 "pavement-toothed" shark, Ccstracion japoiiiciis, in which, as 

 the author showed in a recent number of the " Annotationes 

 Zoologicae," surface furrows are present traversing the yolk, 

 and are best interpretable as reminiscent of holoblastic cleavage. 

 In the peculiar type of early development in CJiiiiKBra, total 

 cleavage is suppressed until about the time of gastrulation, 

 when cleavage furrow^s appear in the region of the lower pole 

 and come to divide the O:^^ into a number of distinct blasto- 

 meres, only one mass of which, however, become enclosed in 

 the yolk-sac of the embryo. The remaining blastomeres, by a 

 process of continued division, provide nutriment for the embryo, 

 via gills and gut. Dr. Dean announced the presence in 

 CJiimcEra of a true archenteric invagination, occurring early and 

 at some distance from the margin of the blastoderm. It is 

 small in size, and has a distinct cellular floor. Its (anterior) 

 dorsal wall was compared to the dorsal lip of the archenteron 

 of sharks, as described by Riickert and others. The ventral 

 wall of the archenteron of modern types of sharks has thus 

 lost its cellular character during the process by which the em- 



