MACKOPOMA MANTELLII. 



35 



Posteriorly, the parasphenoid is broken, but probably passed into 

 the flat bony floor of the parieto-occipital division of the skull, 

 which may either be an extension backward of the parasphenoid, 

 such as exists in the sturgeon, or may be formed by coalescence of 

 the latter with a true basi-occipital. 



Above its spatulate dentigerous part, the basal bone passes up- 

 wards and outwards into strong lateral plates (Plate YIII., fig. I5 

 /) 5 which are concave outwards, and unite with the frontal 

 shield. They represent the ■prefrontals. 



Each prefrontal gives off from its anterior end, just above the 

 rounded extremity of the dentigerous plate, a stout process 

 (Plates YII., fig. 6, YIII., figs. 1 and 3, c?), which passes down- 

 wards and outwards, and ends by a free rounded extremity at the 

 margin of the gape, close to the anterior end of the maxilla. This 

 appears to represent the process of the prefrontal bone with which 

 the palatine articulates in ordinary fishes. 



The interorbital space above the parasphenoid, from the pos- 

 terior margins of the prefrontals as far as a point a little in advance 

 of the junction of the fronto-nasal and parieto-occipital shields, 

 seems to have been devoid of ossifications answering to the orbito- 

 sphenoids and alisphenoids ; but farther back the sides of the para- 

 sphenoid pass indistinguishably into the -pro-otic bones. Each of 

 these is a large plate of bone, rising perpendicularly towards 

 the roof of the skull, which it nearly reaches in front. Further 

 back it sends out two great processes, one superior and the other 

 inferior, at right angles to its own plane. 



The superior process (Plate VIII., fig. 1, e), curving outwards 

 abuts against the under surface of the occipito-parietal shield, close 

 to the middle of its outer margin, and furnishes an articular facet 

 for the proximal end of the hyomandibular bone (Plate VIII., 

 fig. 2, H.M,\ 



The superior process of the pro-otic is separated by a deep oval 

 fossa from the inferior process (Plate VIII., figs. 1 and 2,/), which 

 is a stout plate of bone, convex from above downwards on its 

 outer surface, and ending in front by a free thick edge, represented 

 somewhat too round in PL VIII., fig. 2. The lower incurved 

 edge of this scroll-like plate does not come into contact with 

 the osseous basis cranii, which hereabouts begins to be defective. 



The root of the superior process (PL VIII., fig. 1 e) separates 

 two slit-like foramina which lead into the interior of the skull, and 

 probably gave exit to divisions of the fifth nerve. From the upper 

 and external edge of the inferior process a vertical bar of bone is 

 sent off and, abutting against the superior proc^fess, bounds an oval 

 fossa behind. 



Externally and posteriorly, the pro-otic abuts, by an abruptly 

 truncated and perpendicular face, against another stout lateral osseous 

 mass {g\ which appears to represent the opisthotic and exoccipital. 

 From this three processes pass, one ascending, which lies against 

 the vertical bar (/^) of the pro-otic ; a second external and ascending 

 process (PL VIII., figs. 1 and 4, passes upwards and backwards 

 to unite with and support the supra-scapular part {S, Sc.) of the 

 parieto-occipital shield. The third process (PL VIII., figs. 1 and 4, i) is 



C 2 



