206 ME. W. K. PAEEER ON THE STEUCTUEE AND DEVELOPMENT 



The pharyngobranchial of the fourth arch belongs also to the fifth (fig. 3) ; forking 

 below, it is attached to the apex of both the fourth and fifth. This latter arch has its 

 epibranchial continuous with the pharyngobranchial of the fourth ; it has a flat notched 

 ceratobranchial piece (fig. 4, c.hr, 5), and has no hypobranchial. 



The sectional views further illustrate this stage of nearly ripe embryos of the Dog-fish. 



A vertically longitudinal section (PI. XXXVI. fig. 6) shows how completely the cranial 

 cavity is filled with the brain, and that the mesocephalic flexui-e is obliterated. The 

 teo-men cranii is continued over part of the fore brain ; and the floor of the cranium is 

 one continuous sheet of cartilage, formed by the investing mass behind, and by the 

 united trabeculse in front. The nasal sac (pi) now lies in front of, as well as some- 

 what beneath the fore brain. Beneath and behind the olfactory folds is seen the distal 

 end of the pterygoquadrate bar (j.p^), and below the mouth the distal end of the man- 

 dible {mn). The hyoid crus and base {c.hij, h.hy) and the distal parts of the branchial 

 arches are shown, as also the basihyal and basibranchial, in section {b.hj, b.br). There 

 is but little of the notochord {nc) left ; and a posterior clinoid ridge shows the rudi- 

 ment of the " sella turcica." The pituitary body {]py) is very small ; beneath it the 

 internal carotid is seen entering the cranium. 



In the first of the transverse sections (PL XLII. fig, 5) the eyeballs are cut through, 

 and a view is gained of the height and width of the cranium, built upon the foundation 

 of the trabeculee and their " commissure." The orbitosphenoidal side walls connect 

 the trabecular crest with the superorbital band and the " tegmen cranii." Below the 

 mouth (ni), the oral " labials," the pterygoquadrate bands, the mandibles, and the fore 

 part of the basihyal are cut through {ci.]}g, mn, b.hy). 



In another section further backwards, and somewhat oblique (fig. 6), the hinder part 

 of the eyeball (e) and the fore part of the auditory sacs are shown. The basal cartilage 

 is cut through where the trabeculse have coalesced with the investing mass. The third 

 section (fig. 7) is through the posterior and horizontal semicircular canals {p.sc, h.sc), 

 the upper part of the oblique foramen magnum {f.m), the investing mass, and remnant 

 of the notochord {iv, nc), and the head of the hyomandibular, or " epihyal " {h.m). 

 The thick auditory capsule is seen to be still distinct above from the superoccipital 

 cartilage (so). The passage for the glosso-pharyngeal and vagus (9, 10) is seen below. 



The remarkable extravisceral cartilages (PL XXXVI. fig. 4, ex.br) are shown outside 

 the branchial arches : they are sharp above, and dilated below ; there are four pairs of 

 them ; and the last [e.vs, 9) is very small. 



These cartilages await proper classification; at present they may be bundled up with 



the " labials." 



Fourth Stage. Adult Bog-fish (Scyllium canicula). 



At first sight it might be supposed that a skull without any proper ossification in 

 the adult state would present the greatest diflaculties to the morphologist ; for the 

 various elements of the cranium are here thoroughly soldered together, making a com- 



