334 Prof. Owen's Description of the Lepidosiren annectens. 



of this plate is turned forwards, the apex expands into an ol)lique elliptical 

 plate, having a shallow depression, which receives the pointed anterior ex- 

 tremity of the gelatinous chord. The fibrous capsule of the chord is attached 

 to the margin of the basi-occipital depression, and supports the neurapophyses 

 of the atlas, anterior to which are the corresponding elements of the occipital 

 vertebra itself. These exoccipitals* present the form of thin expanded plates, 

 of a subquadrate figure, concave towards the medulla oblongata, which they 

 defend and embrace, their anterior and inferior extremities being extended into 

 the upper part of the capsule of the fibrous chord, as far as the middle line, like 

 the bases of the ordinary neurapophyses : their upper extremities meet above by 

 an extended superior margin. The basi-occipital is notched below the base of 

 each exoccipital, leaving a foramen for the transmission of a large nerve. The 

 limits of the basi-occipital and sphenoid bones are indicated by a transverse 

 groove. The basi-sphenoid sends upwards, near its outer margin, two low 

 longitudinal vertical plates, which give attachment to the cartilaginous alse 

 of the sphenoid forming the lateral parietes of the cranial cavityf. Between 

 these alse and the ex-occipital bony plates is interposed on each side the large 

 and thick cartilaginous capsule of the organ of hearing:}:, which is extended 

 upwards to the parietal bone, and represents the petrous and squamous ele- 

 ments of the temporal bone. There is no distinct supra-occipital bone or spine, 

 but its place is occupied by the posterior extremity of the parietal^. This is 

 a single symmetrical, lozenge-shaped bone, from the middle line of whose 

 upper surface a longitudinal stout spine is developed, augmenting the surface 

 of attachment of the strong temporal muscles. This spine is continued upon 

 the frontal bone|| as far as its anterior extremity. Both the parietal and 

 frontal bones are ossified, and were anchylosed together in the larger speci- 

 men : the limits of the cranial vertebrae are thus effaced above as they are by 

 the confluence of the occipital and sphenoids below. The anterior part of the 

 frontal is deeply notched on each side. There are no distinct ossified anterior 

 frontals, but the posterior frontals^ are enormously developed, and extend 

 backwards over the frontal and parietal bones as far as the occiput, forming 

 a second bony shield to the skull, analogous to the osseous plates in the 



♦ Tab. XXIII. fig. jl, «'. f These portions are distinguished by dots in Tab. XXIII. fig. 4. 



J Tab. XXIV. fig. 2. *, k. § Tab. XXIII. figs. 4 & 5, A. || lb. /. % lb. mm. 



