LIASSIC ICHTHYOSAURS. 59 



The lacryraal (Tab. XX, 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 npper part of the lacrymal 

 sends a process which fits into a notch of the prefrontal (ij), 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 {l2 x,y and prosquamosal (27');" both are present in 

 Labyrinthodonts. The postorbital is the homologue of the lower division of the post- 

 frontal in those Lacertians (<?. y. Iguana, Tejns, 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 sliglitly 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 Chelotie, the 

 resemblance to those parts of the Mammalian ' temporal bone ' would be close, save 

 that the squamous portion would be removed from the inner to the outer wall of the tem- 

 poral fossa. The prosquamosal holds the place of the temporal fascia in Mammals, and 

 should be viewed as a sclerodermal ossification closing, in Ichthyosaurus as in Lahyrin- 

 thodon, the vacuity between the upper and lower zygomatic arches, such as exists in Croco- 

 dilia. In Ichthyosaurus the prosquamosal (27') is a broad, thin, flat, irregularly-shaped 

 bony plate, smooth and subconvex outwardly, wedged into an interspace between the 

 postfrontal, postorbital, zygomatic, tympanic, and mastoid bones. 



The chief vacuities in the skull are : — In the occipital region (Tab. XXII, fig. 1), the 

 ' foramen magnum ' or neural canal of the occipital vertebra ("), the ' occipito-parietal,' 

 (0^:1), and the 'auditory' (?«) ; on the upper surface (Tab. XIX, fig. 1), the 'foramen 

 parietale ' (/) and the ' temporal fossae ' (t) ; on the sides (Tab. XX), the ' orbits ' (o) 

 and the ' nostrils ' («) ; on the lower surface (Tab. XXI) the ' palato-nares ' (p «), the 

 ' interpterygoid ' («), and the ' pterygomalar ' (y)^ apertures. 



The ' foramen magnum ' is formed by the basi-, ex-, and super-occipitals, the last 

 having a nearly equal share with the exoccipitals ; the basioccipital contributes the least 



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



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

 of this bone in Ictithyosuurus inclined tne 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. 



^ ' Pierygomaxillary ' in Crocodiles and Lizards, ' Anat. of Vertebrates,' vol. i, pp. 15G and 1J7, 

 fig. 98, y. 



