336 PROP. T. W. BRIDGE ON THE MORPHOLOGY OF THE SKULL 



The chondrocranium of Lepidosiren presents several interesting features, and for 

 convenience in description may be divided into (1) an occipito-periotic region, including 

 also the suspensorium ; (2) a central trabecular portion ; and (3) an anterior or ethmo- 

 nasal region. 



The occipito-periotic region consists of a thick basal cartilage underlying the foramen 

 magnum and extending in the floor of the cranial cavity as far forward as the middle 

 of the periotic capsule, where it terminates somewhat abruptly on the cranial surface 

 of the para sphenoid by a well-defined transverse margin, immediately behind the 

 position occupied by the pituitary body (PL XXVIII. fig. 4, and PL XXIX. fig. 19). 

 A slight concavity on the upper surface of the parasphenoid is the only indication of a 

 pituitary fossa (PL XXIX. fig. 18). Into the axis of the basal cartilage (fig. 4) 

 the filiform intracranial portion of the notochord (no.) is prolonged, and may be 

 traced in sections nearly as far forward as the cartilage itself extends. On its ventral 

 surface the basal cartilage is in close relation with the hinder part of the parasphenoid, 

 while behind, where it becomes continuous with the chordal sheath, the cartilage helps 

 to support dorsally and laterally the bases of a pair of ossified " basi-dorsals " forming 

 the lateral elements of the first neural arch (PL XXVIII. figs. 1 and 4, n.a.). 

 Anteriorly to the " basi-dorsals " are the two exoccipital bones (figs. 1 and 4 ; 

 PL XXIX. fig. 19, eo.), each of which consists of an inwardly-curved basal plate 

 resting on the upper surface of the basal cartilage, but separated mesially from its 

 fellow by an intervening tract of the same cartilage. Each bone then curves upward 

 (fig. 4), forming the side-walls of the hindermost section of the cranial cavity, 

 and terminates dorsally in a thick plate of cartilage which occupies the supraoccipital 

 region, beneath the hinder part of the fronto-parietal bone, and forms the actual roof 

 of the cranial cavity in this region (fig. 19). In a transverse section of the cranium 

 taken through the two exoccipitals (fig. 19), it is seen that the dorsal cartilage extends 

 downward for some distance on each side into the substance of the exoccipital, 

 dividing the latter for a portion of its extent into an outer lamina of bone, which 

 dorsally abuts against the outer margin of the fronto-parietal, and an inner lamina 

 ending above in the dorsal cartilage. Each occipital (fig. 4) is deeply constricted in 

 the centre, owing to the presence of a deep notch in its anterior and posterior margins, 

 the anterior notch forming the posterior boundary of the foramen for the Vagus nerve 

 (x.), the posterior transmitting the roots of the Hypoglossal or first spinal nerve 



(sp. 7l'). 



The basal cartilage and the cartilage of the supraoccipital region, though otherwise 

 distinct, become continuous in front of the foramina for the Vagus nerves with the 

 laterally-bulging cartilage of the periotic capsule, and through the latter with the 

 proximal portion of the suspensorial cartilage (fig. 4). 



Externally and dorsally (fig. 1), the periotic capsules are completely hidden from 

 view by the lateral extension of the fronto-parietal, except for a narrow tract on each 



