The Anatomy of Chlamydoselachus 355 



The postorbital process closely approaches the palatoquadrate but does not articulate 

 with it. The orbital process of the palatoquadrate is unusually large and projects far 

 into the deeper portion of the orbit, where it articulates with a large facet on the ventral 

 edge of the anterior wall of the orbit. The orbital process forces the eyeball away from 

 the medial wall of the orbit. These relations must change considerably when the pharynx 

 is expanded, on account of the spreading of the jaws posteriorly and the shifting of the 

 angles of the jaws ventrad (note the space between palatoquadrate and cranium in Text- 

 figure 84, p. 429). The only articulation of the palatoquadrate with the cranium is by 

 way of the orbital process, which is very loosely attached to the cranium. 



The eyestalk of Chlamydoselachus is a slender rod of cartilage which projects from 

 the anterior edge of the trigemino-pituitary fossa and curves around the posterior surface 

 of the capsular sheath of the orbital process of the palatoquadrate (Figure 2, plate I; 

 Figures 5 and 6, plate II). Its distal end has a sliding articulation with the medial surface of 

 the eyeball, without being attached to it. According to Gegenbaur (1872) the eyestalk 

 of the plagiostomes does not belong genetically to the eye, neither does it, except in its 

 basal portion, belong to the chondrocranium. In all the plagiostomes, the basal portion 

 of the eyestalk is of firmer tissue than the remainder of the stalk, which is always of softer 

 tissue than the chondrocranium. Gegenbaur suggested that the eyestalk (excepting 

 its basal portion) might be a part of the visceral skeleton that had secondarily acquired 

 relations with the eyeball. AUis (1923) cites Dohrn's suggestion that it might represent 

 a part of a premandibular visceral arch, and recalls his own earlier suggestion (Allis, 

 1914, p. 365) that ''the eyestalk is a modified branchial ray or rays, of a mandibular or 

 premandibular arch, that has secondarily acquired relations to the eyeball." While such 

 explanations are highly speculative, an origin from a branchial ray of the mandibular 

 arch seems the most plausible. That the eyestalk originated from some pre-existing 

 cartilaginous structure seems indicated by this statement from Allis (1914, p. 347): 



The eyestalk is certainly a retrograding and archaic structure, as its varying importance 

 and wide distribution clearly indicate, and it seems certain that it could not have been de- 

 veloped independently, merely as a support to the eyeball, a function it so inefficiently 

 fulfils except in certain rays (Harman). And that it was developed as a point of attachment 

 for the recti muscles seems improbable because it actually fulfils that function, so far as I can 

 find, only in Chlamydoselachus (Hawkes, 1906) and possibly in Zygaena. 



At the bottom of the endolymphatic fossa (e/. Figure 1, plate I) are four apertures, 

 two on each side, described by Goodey (1910.1) and by Allis (1923, p. 155). Each anterior 

 aperture is a foramen ductus endolymphaticus, or aqueductus vestibuli, and affords 

 passage for the ductus endolymphaticus. Each posterior aperture leads directly into the 

 perilymphatic cavity of an ear, and is the so-called fenestra ovalis of Scarpa, or the fenestra 

 vestibuli cartilaginei of Weber. In the natural state this aperture is closed by a membrane. 

 In Chlamydoselachus and in Mustelus, the fenestra vestibuli lies immediately above the 

 apex of the posterior membranous semicircular canal of the ear. 



