OF NEW SOrTH WALKS. '^2\ 



in the series of Elasmohranchii, induced me to regard the investi- 

 gation of the dentition of the embryonic stages of that genus 

 as a matter of high interest and importance. Unfortunately, I 

 could procure no Heterodontus embryos, but this proved not to 

 be an insuperable obstacle, as the examination in young specimens 

 of as much as 225 mm. (8*9 in,)* in length of the form of the teeth, 

 which differ very markedly from those of the adult, affords us a 

 glimpse into the genealogical connection of this shark with the 

 other Plagiostomata A glance at the teeth of a young H. PhilUpi 

 (PI. 24, fig. 14), magnified about five diameters, is sufficient to 

 establish the great similarity between the dental armature of the 

 young Heterodontus and that of the Notidani**. This preparation 

 (fie 14) shows us further that at this age (225 mm. in length) 

 not more than seventeen series (i.e., vertical rows) are developed 

 in the upper jaw, and thirteen in the lower. In both jaws only 

 three horizontal rows of both series are to be seen, the others 

 come into view after the mucous membrane of the palate has been 

 dissected off. The teeth situated in the very front of the lower 

 jaw have three almost similar pointed cusps, while those situated 

 further back have five points. (Vide fig. 15, PI. 24).*** 



The teeth of the middle row are more or less symmetrical, and 

 in the posterior (reserve) teeth of this row the central cusp is 

 the largest, while the two outer take the form of inconspicuous 

 tubercles. The teeth of the lateral rows are also longer, in pro- 

 portion to their height, than those of the middle rows. On a 

 closer inspection we see that the two anterior cusps of the lateral 

 teeth are more perpendicular than the others, a feature which is 

 met with in some fossil Notidanus teeth. (Vide Agassiz Poissons 



"^ Mr. Macleay believes that this specimen had only emerged from the egg one or two 

 days. 



** This circumstance gains greater significance, when we consider that the investiga- 

 tion of the brain of the Heterodontus leads us to a similar conclusion in regard to the 

 connections between those two genera. The brain of If. PhilUpi stands very near the 

 general fundamental form of the vertebrate central nervous system, and as such, resembles 

 the brain of the Notidani, of the genera Acanthias and Scymmus. (Beitrage zur var 

 gleichanden Neurology, I., p. 48.) 



*** The difference between the teeth of the youne Heterodontus and that of the adult 



has already been noticed by Mtiller and Henle as follows: — "Zahne, inder Mitte, 



klein und spitz, aus drei bis luni von der Basis gegen die Spitzen ausseinandertretenden 

 Zacken gebildet, von denen die mittlere am grossten ist, die bei alten Thieren abgesch- 



liffensind; .'..(MuUer und Henle. Systematische Beschreibung der Plagiostomen, 1841 



pag 76. 



