24 



FREDERICK TILNEY AND LUTHER F. WARREN 



d'Erchia ('96) 109 differentiated in Pristiurus the same elements 

 as in Notidanus, but in Torpedo he found that the epiphyseal 

 complex was entirely wanting. He further observed that the 

 development of the velum transversum occurred much earlier 

 than the pineal organ. Minot ('01) 277 maintained that an actual 

 paraphysis does not develop in selachians. In comparing the 

 pineal regions of cyclostomes with selachians, the most striking 



Po- 



Fig. 2 Schematization of pineal region in Selachians, according to Studnicka 

 1905. 



Ls., lamina terminalis; Pf., paraphysis; V. velum transversum; Ds., dorsal 

 sac; Po., pineal organ; St., stalk of pineal organ; Ch., commissura habenularis; 

 R., recessus pinealis; Cp., commissura posterior; Sch., pars intercalaris posterior; 

 Prox., proximal portion; Tp., tractus pinealis. 



differences appear to be in the extreme development of the 

 parapineal and pineal organs in Petromyzon and allied forms, 

 while the parapineal organ is absent in selachians. Further- 

 more, the absence of any distinct velum transversum in cyclo- 

 stomes makes the presence of a definite paraphysis extremely 

 doubtful, while the velum transversum in selachians differen- 

 tiates very clearly a fairly well formed paraphysis. The pineal 

 region in Elasmobrdnchs is much shorter than in Petromyzon. 



