LIASSIC FORMATIONS. 
101 
to form the hind boundary of the palato-naris {p n). The outer margin contributes to 
bound the pterygomaxillary vacuity (y) posterior to its junction with the maxillary, along 
which it extends to opposite the fifth upper tooth or thereabouts, counting forwards, 
and there terminates in a point. 
The malar (PI. XXIV, fig. 1, 2C) is a long and slender, moderately bent bar of bone, 
commencing anteriorly in a point wedged between the lacrymal and maxillary, thence 
receding to form the lower boundary of the orbit, and expanding to be partially over- 
lapped by the postorbital bone (i2'), behind which it terminates as a wedge between that 
bone and the zygomatic. 
This bone (ib., 27) is subquadrate, its vertical equalling its longitudinal extent ; the 
angles are more or less produced. The upper anterior one is wedged between the pro- 
squamosal (27') and postorbital (i2x) bones ; the lower anterior one underlaps the end of 
the malar; the front border articulates with both malar and postorbital, the upper 
border with the prosquamosal ; the hind border is rounded and concave, forming the 
fore part of the ‘meatus auditorius externus’ (PI. XXVI, fig. m)', the hinder half of 
the under surface of the zygomatic expands, and is slightly excavated to articulate with 
the outer and upper part of the expanded articular end of the tympanic (28')- 
The lacrymal (PI. XXIV, fig. 1, 73) forms the lower two thirds of the anterior 
border of the orbit ; it sends off from the middle and inner part of this border a short 
process protecting the lacrymal orifice. The bone contracts vertically as it approaches 
the nostril, of which it forms the hind concave border. The upper part of the lacrymal 
sends a process which fits into a notch of the prefrontal (14), anterior to which it joins 
both the prefrontal and the nasal. Anteriorly, it unites in some species with the maxillary 
(ib., fig. 1), in most with both maxillary and premaxillary (ib., fig. 2) ; its lower and 
longest margin articulates with the maxillary and malar. 
The two supplemental skull-bones in Ichthyosaurs, which have no homologues in 
Crocodiles, are the postorbital (12 a;,)^ and prosquamosal (27') both are present in 
Labyrinthodonts. The postorbital is the homologue of the lower division of the post- 
frontal in those Lacertians {e. g. Iguana, Tejus, Ophisaurus, Anguis) in which that bone 
is said to be divided. The postorbital most resembles a dismemberment of an ascending 
process of the malar ; its lower end overlaps and joins by squamous suture the hind end 
of the malar ; whence it slightly expands, rising to the middle of the back of the orbit, 
thence, gradually contracting as it curves upward and forward, it articulates with the pro- 
squamosal (27') and postfrontal (12). 
Were the prosquamosal (27') connate with the zygomatic (27) as in Clielone, the 
1 Described as “ apparently a distinct and peculiar bone ” of tbe orbit in the ‘Report ’ for 1839. 
~ This is termed “ squamosal” in tbe Lectures above cited (1838), p. 392. The recognised distinctness 
of this bone in Ichthyosaurus inclined me in 1839 to view the zygomatic and squamous parts of the 
temporal bone of anthropotomy as essentially distinct elements ; a view which subsequent extensions of 
comparison enforced me to abandon. 
