108 



MR. E. PHELPS ALLIS, JUN., ON 



in both these fishes, lie, at this age, in the line prolonged of the 

 trabecular plate, and each lateral end of the narrow plate of the 

 one (Pristiurus), and each postero-lateral corner of the broad 

 plate of the other (Acanthias), turns dorso-posteriorly as a band- 

 like process which passes dorsal to the nervus olfactorius and 

 dorsal to the nasal sac, and, fusing posteriorly with the anterior 

 ends of both the ethmoidal and sphenolateral cartilages, forms 

 the dorsal boundary of the opening that transmits the nervns 

 olfactorius; the sphenolateral cartilage quite certainly not taking 

 any part in the formation of the actual boundary of the opening. 

 This opening will be hereafter called the fenestra olfactoria in 

 both these fishes, notwithstanding that it is said by SewertzofF to 

 be later subdivided into a foramen olfactorium and a basal com- 

 municating canal in Acanthias, and to represent the definitive 

 foramen olfactorium in Pristiurus. 



The nasal capsule of either side, as shown in Sewertzoff's 

 figures, is represented by a curved plate of cartilage which 

 encircles the basal portion of the anterior and lateral surfaces of 

 the nasal sac and projects ventrally from the ventro-lateral 

 surface of the rostral plate. It is completely fused, by its dorsal 

 edge, with the rostral plate, but its position, suggests that it is 

 primarily an independent cartilage secondarily fused with that 

 plate. Parker (1876) shows the nasal capsule in a somewhat 

 different condition in SeyUiurn, but the figures given by him are 

 difficult to comprehend and not readily compared with those 

 given by SewertzofF. The capsule, however, as shown by both 

 these authors in these several fishes, lies ventro-lateral to the. 

 trabecular and rostral cartilages, and where, in these or others 

 of the Selachii, the capsules of opposite sides have been pressed 

 together so as to meet in the median line and there be separated 

 only by a septum nasi, that septum lies ventral to the trabecular, 

 as shown in Parker's sectional views of Scyllium and in Gegen- 

 baur's sectional view of Mustelus. The trabecular, in each of 

 these latter fishes, curve dorsally above the nasal sacs and are 

 furnished with a deep median subethnioidal keel which extends 

 ventrally between the sacs and is shown, in each case, as largely 

 fenestrated in its middle portion. Parker (1876, p. 207) appa- 

 rently considered this internasal septum to be an upgrowth of 

 the trabecular cartilage, but Parker and Bettany (1877) say that 

 the trabecular plate gradually rises to the level of the nasal roof 

 and there suddenly narrows and is confiuent with the internasal 

 septum, this seeming to mean that the septum lies ventral to 

 the trabecular, as 1 consider it to do. This difference in interpre- 

 tation, by these two authors, of identical conditions is apparently 

 due to Parker having considered the trabecular to be continued 

 forward in the ventral edge of the subethnioidal keel, while 

 Parker and Bettany consider them to be represented in the 

 plate from which that keel descends, that is in the trabecular 

 crest of Parker's descriptions. 



A ventro-lateral trabecular process, lying antero- ventral to 



