LIASSIC FORMATIONS. 101 



to form the hind boundary of the palato-naris (p «). 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, 2G) 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 (12'), 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 (l2x) 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 auclitorius externus' (PI. XXVI, fig. 1, 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 x,) 1 and prosquamosal (27') ; 2 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 Chelone, the 



1 Described as " apparently a distinct and peculiar bone " of the orbit in the ' Report ' for 1839. 



3 This is termed "squamosal" in the Lectures above cited (1858), p. 392. The recognised distinctness 

 of this bone in Ichthyosuurus 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. 



