326 



a relatively small imperfection in the ventral portion of the nasal 

 capsule. The nasal incisure (Nasallücke) of Gegenbaur's descriptions 

 of this fish is certainly not the nasal fontanelle, as I have defined it, 

 and it can not be the ectethmoidal chamber because that chamber 

 is represented in the Y-shaped canal above referred to. 



The basal communicating canal of Gegenbaur's descriptions is, 

 in Heptanchus and Acanthias, the only fishes in which I have been 

 able to examine it, a perforation of the planum internasale of Gaupp's 

 nomenclature and not of the solum cavi praecerebralis, this being in 

 accord with Sewef.tzoff's opinion that this perforation represents 

 a portion of the large fenestra olfactoria of embryos that has been 

 cut off from the definitive foramen olfactorium by the growth across 

 the fenestra of a longitudinal bar of cartilage. The perforation, in 

 the adults of both these fishes, thus leads definitely from the inter- 

 fenestral portion of the cavum cranii and not from the cavum 

 praecerebrale. In Heptanchus it opens ventrally directly into the 

 ectethmoidal fossa, and must accordingly perforate the interfenestral 

 cartilage (planum internasale) lateral to the line of origin of the sub- 

 nasal plate from the cranial wall. In Acanthias it opens on the ven- 

 tral surface of the chondrocranium mesial to the ectethmoidal chamber, 

 and must accordingly perforate the interfenestral cartilage mesial to 

 the line of origin of the subnasal plate from the cranial wall. That is: 

 in Heptanchus the subnasal plate apparently has its origin from the 

 cranial wall along the line of the ventro-mesial edge of the fenestra 

 olfactoria of embryos, while in Acanthias, it has its origin along the 

 line of the ventro-mesial edge of the definitive foramen olfactorium. 

 This slight difference in the- line of origin of the subnasal plate from 

 the cranial wall would fully explain the different conditions in the two 

 fishes, but it is evident that the fact that such a difference actually 

 exists must be established by further study of the development of this 

 part of the cranium. 



In the Carchariidae and ScyUiidae the cavum praecerebrale is 

 represented in all, or a part, of the space enclosed between the carti- 

 laginous bars that form the three-limbed rostral process of these 

 fishes, and, in the only ones of these fishes that I have been able to 

 examine, Galeus and Mustelus, there is an ectethmoidal chamber but 

 no perforation of the planum internasale. And that the perforations 

 of this latter plate, one on either side, found in Heptanchus and the 

 Spinacidae, can be represented in the Carchariidae and ScyUiidae in 



