322 MK. J. S. BUDQETT ON THE STKUCTUUE 



outer portion of the supraorbital bar of cartilage at the point wlierc it is fused witli 

 the olfactory capsule (PI. XXXIII. fig. l,f.oL). Just anterior to this the capsule 

 itself is penetrated by the same nerve. The olfactory capsules are very large and 

 thin-walled, oval-shaped, and perfoiated anteriorly by a crescent-sliapcd nasal aperture 

 (PI. XXXIII. fig. 1). They are separated from one another to some extent anteriorly 

 by a deep groove which ends above a spade-shaped rostral prolongation of the ventral 

 portion of the nasal septum. 



The ventral side of the olfactory capsule bears a facet for the articulation of the 

 anterior end of the palato-quadrate bar of cartilage (PI. XXXIII. fig. 1, P.Qu.art.). 



The general form of the primordial cranium at this stage resembles very much that 

 of the Selachii, especially the Scylliidae, but in the absence of cartilage in the middle 

 portion of the cranial walls, and the size of the great fontanelle, there is some 

 similarity to the Urodela, The bridge of cartilage across the supracranial fontanelle 

 i>cems to indicate a former roofing of the cranial cavity by cartilage, for, later, this 

 bridge is represented by the small plate of cartilage noted by Pollard, and finally 

 disappears. The absence of cartilage immediately over the pineal gland seems to 

 indicate that the cranial roof was formerly perforated by a parietal foramen. 



The Visceral Arches. — The falatoquadrate bar is a pyramidal mass of cartilage, the 

 anterior corner of which is drawn out into a vertically flattened bar, which anteriorly 

 becomes almost circular in section, and finally flattened horizontally to form the 

 articulation with the facet on the ventral side of the nasal capsule (PI. XXXIII. 

 fig. 1, P.Qu.art.). The quadrato-mandibular articulation is saddle-shaj)ed, and it is 

 in the region of this articulation that the palato-quadrate bar has its greatest transverse 

 diameter (PI. XXXIII. figs. 1, 2, 3, P.Qio., Mk.). 



Behind this articulation the bar narrows to a point, which is concave vertically. 

 This concavity rests upon the lower limb of the hyomandibular cartilage. The palato- 

 quadrate bar receives no direct support from the cranial wall ; the articulation is 

 therefore hyostylic. 



The mandibular bar has a similar pjramidal shape posteriorly, while its anterior 

 portion forms a more regularly cylindrical rod than the palato-quadrate bar, and tapers 

 to the symphysis with its fellow (PI. XXXIII. figs. 1, 2, 3, Mk.). 



The hyoidean arch, consisting of hyomandibular, stylohyal, ceratohyal, and hypohyal 

 cartilages, articulates with the facet already mentioned on the underside of the lateral 

 ridge of the auditory region (PI. XXXIII. figs. 1, 2, 3, Pt.r.). 



The hyomandilular consists of two rounded masses connected by a slender curved 

 rod bearing upon its posterior convex surface the anterior end of a rod of cartilage 

 segmented into two fairly equal portions, which constitute the skeletal axis of the base 

 of the external gill (PI. XXXIII, figs. 1, 2, 3, Op.). This rod persists as the small 

 nodule of cartilage on the inner side of the opercular bone, which has been before 

 noted by Van Wijhe and Traquair. 



