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 (Hydromedusd) 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 Reptilian, 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. n, 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 J, 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 thp 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 (ib. 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 olfactory 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. 



f The urinary bladder and intromittent organ, e. g. : the modification of the feathers in 

 the Struthionidee is a degeneration of a peculiarly ornithic character ; hut not, therefore, an 

 approximation to the hairy covering of mammals. 



X In the emeu (Dromaius ater) at 14, fig. 1. pi. 39. Zool. Trans, t. iii. : and in the casso- 

 wary at h, fig. 3, taf. i. in Hallmann's ' Vergleichende Osteologie des Schlafenbeins.' 



