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cartilage. If it were to become entirely closed by such ingrowth, the 

 conditions here found in ganoids would arise. 



This incisure in the subnasal plate of Chlamydoselachus has related 

 to it an imperfection in the chondrification of the nasal capsule and 

 also a space, or chamber, that lies dorso-posterior to the incisure, between 

 the tough lining membrane of the nasal cavity, the subnasal plate and 

 the antorbital cartilage. These three distinctly different structures, 

 in Chlamydoselachus, are all included in the so-called nasal fontanelle 

 (Nasallücke) of Gegenbaur's descriptions of certain of the selachians 

 described by him, while in others that name is apparently apphed to 

 one or the other of the three. Not wishing, from this study of the 

 adult alone, to use the term fenestra choanalis in this connection, 

 I propose to employ Gegenbaur's term, nasal fontanelle, to designate 

 the subnasal incisure and the related imperfection of the nasal capsule, 

 and to call the related chamber the ectethmoidal chamber. This chamber 

 communicates with the external surface of the chondrocranium by 

 two openings; the one a membranous or cartilaginous foramen on the 

 ventral or ventro-lateral surface of the chondrocranium which transmits 

 a branch of the rostral branch of the external carotid artery together 

 with a branch of the anterior facial vein, and the other a short canal 

 which perforates the antorbital cartilage and transmits into the orbit 

 the anterior facial vein above referred to reinforced by venous vessels 

 coming from the nasal capsule. The chamber is accordingly an en- 

 largement of the middle portion of the orbito-nasal canal of Gegen- 

 baur's descriptions, that canal being wrongly said by Gegenbaur to 

 transmit a branch of the ramus ophthalmicus trigemini (Allis, 1901). 



In Gegenbaur's descriptions of the selachians investigated by 

 him, it is difficult to definitely identify the nasal fontanelle and 

 ectethmoidal chamber. Both structures are apparently found in 

 Hexanchus, and probably also in Squatina (Khina) and Cestracion 

 (Heterodontus), and I find them both in Galeus. In Heptanchus the 

 fontanelle can not be recognised, but the ectethmoidal chamber is 

 represented in the fossa (Bucht) that Gegenbaur describes as re- 

 presenting the entire so-called fontanelle (Nasallücke). The ecteth- 

 moidal chamber has here evidently been shut off from the nasal cavity 

 by the more complete chondrification of the nasal capsule, and its 

 ventral opening has been so greatly enlarged that the chamber has 

 become a fossa. In Acanthias, I find the chamber reduced to a simple 

 Y-shaped canal, while the fontanelle is apparently represented in 



