MIC HO DON. (J I 



marked on their outer face by a sparse series of large openings. No superficial 

 bones have been clearly observed in advance of the frontals covering the narrow 

 ethmoidal region, which seems to consist of a single vertical mesethmoidal plate 

 (me.) with a thickened anterior margin, which is expanded into a rounded lobe on 

 each side in the lower half. This element is triangular in shape, ending abruptly 

 behind within the anterior third of the orbit, and bounded below by the para- 

 sphenoid and vomer, which extend along its posterior and its anterior portion 

 respectively. The parasphenoid (pas.) is relatively large, but its hinder portion 

 beneath the cranium is obscure in all the fossils. Where crossing the orbit the 

 bone is slender, but it bears a thin laminar vertical keel (/»•.) below, with a free 

 wing extending forwards to the hinder border of the mesethmoid region. Where 

 it underlies the mesethmoid in front it also expands above into a low thin vertical 

 lamina. The single vomer (vo.) is widened at its oral face to support the dentation, 

 and its upper median keel for union with the mesethmoid is low, while it extends 

 only slightly further forwards than the latter element. No cheek-plates have been 

 observed, but there are often traces of an ossified sclerotic ring (PI. XIV, fig. 1). 



The mandibular suspensorium is much inclined forwards to the small obliquely- 

 cleft mouth. The hyomandibular (PI. XIV, fig. 10, Jim.) is a deep narrow lamina 

 of bone, slightly curved forwards on its long axis, with a rounded expansion behind 

 in its upper part for the support of the operculum. Its outer face in the upper 

 half is strengthened by a curved longitudinal ridge with its concavity forwards. 

 The quadrate and the pterygo-palatine have not been observed, but must have been 

 comparatively delicate plates. The nature of the maxilla, if present, remains 

 uncertain, but there are sometimes traces of a thin toothless bone flanking the 

 vomer, which may represent a maxillary element (e.g., Mus. Pract. Greol. no. 28352, 

 and B.M. no. 44.844). The premaxilla (PI. XIV, fig. 1) is a long slender rod 

 tapering upwards, gently arched forwards to leave a gap between itself, the vomer, 

 and the mesethmoid, and in contact with the latter at its upper end. Its oral 

 border bears two smooth chisel-shaped teeth, of which the inner is the larger; 

 while its posterior or outer border seems to be notched where there may have been 

 the narial opening. The reduced dentary bone (PI. XIV, fig. 6) closely resembles 

 the premaxilla in size and shape, but its two smooth chisel-shaped teeth are 

 relatively larger. The inner of the two teeth is again the larger. This bone 

 (PI. XIV, fig. 3, (/.) rests as a splint along the front margin of the splenial which 

 forms the greater part of the mandibular ramus. The latter stout tooth-bearing 

 bone is exposed as a wide smooth band on the outer face of the jaw below the oral 

 border (spl.) ; and its hollowed lower portion, which would be occupied by the 

 meckelian cartilage, is covered for the greater part of its extent by a large 

 elongate-triangular angular plate («<j-). 



The tritoral teeth of the vomer and splenial bones are smooth, and when 

 unworn usually exhibit a shallow apical pit, of which the margin may be slightly 



