DEVELOPMENT OF TlIE SKULL IN THE UKODELES. 175 



the jaw. The splenial (sj)!) is smaller, but large also ; it reaches, as in Menopoma 

 (PI. XXXVIII. fig. 3, spl), nearly to the angle, inside ; it carries teeth along its fore 

 half, and is thin all the way, ending a little behind the symphysis, where Meckel's 

 cartilage {mk) is exposed. 



B. The Endocranium of Sieboldia. 



The chondrosteous inner skull of this species is very massive ; about half of tlie 

 cartilage of which it was composed is ossified. It is much hidden by the investing 

 plates ; but its parts are largely traceable notwithstanding. 



The occipital arch is like a low hour-glass, the foramen magnum (PI. XXXVII. 

 fig. 2,f.m) is circular below and narrow above; it has cartilage bounding it in both 

 places. The condyles (oc. c) are large, oval, and look obliquely inwards and backwards ; 

 between them, in the opening, there is a concave joint for the odontoid rudiment. 

 Where the cincture is pinched in, there we see a common passage for the ninth and 

 tenth nerves (fig. 1, LX, x). Each exoccipital bone [e.o) just faces the capsules of the ears 

 with a hoUow bony plate, answering (partially) to both the opisthotic and epiotic. 

 Above, the supraoccipital synchondrosis is covered with the parietals (PL XXXVII. 

 fig. 2, p) ; below, the basal cartilage is floored with a compound plate, formed by 

 fusion of the parasphenoid with the exoccipitals. 



In front the prootics (PI. XXXVI. fig. \, pr.o) occupy a rather small part cf the 

 front face of the capsules, which are unossified to a great degree across their middle 

 part ; in the postero-external face below we see the large oval stapes (PI. XXXVII. 

 figs. 1, 2, St), which is ossified and ossifies half the columella (j).hll). The shallow cranial 

 barge has its low sides ossified in the orbital region ; each bony tract, or sphenethmoid 

 {sp.e), runs halfway from the optic to the oval foramen (ii, v), and runs three fourths of 

 the way to the wings of the ethmoid, where it stops abruptly. 



The actual roof and floor are extraneous ; for the " tegmen cranii " only exists as a 

 rudiment before and behind, and the basal plate (parachordal passing into pro- 

 chordal) is in two widely separated moieties; in front the " intertrabecula " only 

 exists as an internasal conjugation. 



The nasal region {na) is unossified ; it is very broad, is confluent with the dilated 

 cornua trabecule, the form of which has been transferred to the vomero-palatine bones 

 (PI. XXXVII. fig. 1, v.pa). In front the premaxillaries till in the large semicircular 

 notch between the cornua and the nasal roofs. In the angle between the maxillary and 

 prefrontal each nasal roof is exposed ; it is gently convex, and has an elliptical shape ; 

 it does not reach the maxillary, the dyke between is filled up with a fibrous mat. 



c. The Visceral Arches of Sieboldia. 

 Behind, the nasal sacs appear larger than they actually are ; for the large, thick, 

 arcuate ethmopalatines {e.pa) have followed their curve, and are confluent with their 



