224 REPORT—1846. 
in that reptile, as the prefrontals are in fishes, chiefly concerned in closing 
the anterior end of the cranial cavity, in giving exit to the olfactory nerves, 
in suspending the palatine arch, in connecting the vomer with the nasal ver- 
tically, and the nasal with the frontal and lacrymal horizontally, repeating in 
the crocodile for the latter purpose the development of the upper or horizontal 
plate which had almost or entirely disappeared in some of the intervening 
forms of reptiles. In most chelonians this portion of the prefrontal coalesces 
or is connate with the short nasal: but I have found the instructive exception 
presented by the existing freshwater tortoise (Hydromedusa) of the persistent 
suture between the nasals and prefrontals, repeated in two fossil chelonians 
(Chelone planiceps and Chelone pulchriceps)*. 
Proceeding in the ascensive track of the homologies of the prefrontals, 
I have selected from the class of birds the skull of the ostrich (figs. 8 and 23), 
the representative of an aberrant order, in which every deviation from the 
type of the class that has been supposed to tend towards the Mammalia, tends 
equally or more towards the Reptilia+, and in which, conformably with the 
lower development of the respiratory system, the original sutures of the 
cranium, or in other words, the signs of the vertebrate archetype on which it 
is constructed, are longest retained. Were we to cut off the corresponding an- 
terior angles of the frontals, no. 11, to those supposed to represent in mammals 
the bones we are in quest of, we should have even fewer of their characters 
than in the higher class alluded to, because the descending orbital plate is 
less developed, and the frontal, though its general size is much augmented, 
retains more of its oviparous horizontality as an expanded spine or roof-bone 
of the cranium. 
There is a large bone (fig. 23,73) bounding the anterior border of the orbit, 
and from which, as we have seen in the parrots, ossification sometimes extends 
backwards along the inferior contour of the orbit to the postfrontal. But this 
bone, besides its repetition of the connections of the lacrymal in the fish and 
crocodile, resting as in the latter animal upon the true malar bone, is either 
perforated or grooved by the lachrymal duct, which it defends in its course 
from the eye to the nose, and has none of the essential characteristics of the 
prefrontal. But we see on the exterior of the skull of the ostrich and other 
struthious birds, a distinct rhomboidal plate of bone interposed between the 
frontals and nasals, precisely in the situation in which the upper surface of 
the coalesced prefrontals appears in the skull of the frog and other anourous 
batrachians. In a nearly full-grown ostrich’s skull, I removed the left fron- 
tal, nasal, lacrymal and tympanic bones, and the zygomatic arch, as in fig. 8, 
and found the facet in question to be the upper and posterior expanded 
surface of a large irregularly subquadrated compressed bone (7d. 14), consist- 
ing of two vertical compact plates coalesced at their periphery, and including 
a loose cancellous texture. The upper and posterior expanded surface of the 
bone extends a short way back beneath the frontals, descends and closes the 
anterior aperture of the cranium, and sends out from each side a plate of 
bone which arches over the o!factory nerves and forms the canals by which 
they are conducted along the upper part of the orbits. The anterior and upper 
surface of the bone again expands (at 14!, figs. 8 and 23), and there sustains, 
and is covered by, the nasal bones, and again overarches, and is sometimes 
* Report on British Fossil Reptiles, Trans. Brit. Assoc. 1841, pp. 169, 172. : 
+ The urinary bladder and intromittent organ, e. g.: the modification of the feathers in 
the Struthionide is a degeneration of a peculiarly ornithic character ; but not, therefore, an 
approximation to the hairy covering of mammals. 
+ In the emeu (Dromaius ater) at 14, fig. 1. pl. 39. Zool. Trans. t. iii.: and in the casso- 
wary at f, fig. 3, taf. i. in Hallmann’s ‘ Vergleichende Osteologie des Schlifenbeins.’ 
