OF GALLINACEOUS BIRDS AND TINAMOUS. 173 



ient but most important metamorphoses through which the skull passes, and passes so 

 rapidly, before it attains to its adult condition. 



The first thing to be noticed is, that Hemipodius does not agree with the Quail or 

 the Ptarmigan in having the occipital plane nearly at right angles with the base of 

 the skull; but with the Pigeons, Sand-Grouse, and Plovers, in which the occipital 

 plane tends to become almost coincident with that of the basis cranii (Pis. XXXIV.- 

 XXXVII.). 



It agrees with these last-mentioned birds, moreover, and differs from the former, 

 the true Gallinacece, in a certain smooth, rounded, graduating occipital region, the 

 GallincB having this part abrupt and fiat (PI. XXXVI. fig. 8). 



The cartilaginous scaffolding of the occiput is freely developed in the Fowl-tribe, 

 both in its typical, subtypical, and aberrant genera; but in this osculant form there is, 

 as in the Pigeons, a deficiency of this substance, and a retention of the primordial 

 membranous brain-sac as the only coA'ering of the cerebellum for a certain space 

 directly over the " foramen magnum." 



This space exists in typical Pigeons, in the Goura, and in the Dodo, even in the 

 adult skull, as a neat, round hole, like a pin-hole : in very aged individuals of the 

 Wood-Pigeon (Columba paluvibus) it becomes at times walled in with bone ; but this 

 small central tract never went through the cartilaginous stage. 



This " middle occipital fontanelle " (PI. XXXVII. fig. 6) is not insulated in some em 

 bryos of the Common Pigeon {Columba livia) at the time of hatching ; indeed there is 

 great variety in this bird as to the degree in which the occipital cartilage developes the 

 insulating process on each side, to close in above the spinal marrow. 



Very soon, however, the fontanelle is completed by bone, if not by cartilage. 

 ' But in the little Passerine Ground-Dove (ChamcBopelia passerina) the lower part of the 

 mid-occipital region exists as a triangular membranous space formed by the dura 

 ■mater within, and by an extension of the periosteum without. This is also the case 

 in the smaller " Grallse," e. g. Tringa cinclus and Charadrius hiaticula ; but these have, 

 like their proper congeners, a pair of lateral occipital fontanelles, to be spoken of 

 hereafter. 



This imperfectly insulated space exists in Hemipodius. In H. varius I find it as open 

 below as in Chamaopelia passerina, but partly filled up on one side, and bridged across 



beneath in H. ? (PI. XXXIV. fig. 2), the specimen from the Gardens of the Society '. 



There is in birds a very elegant, deep, semicircular fossa which follows the curve of the 

 great anterior semicircular canal, being indeed formed by the projection of that canal 

 inwards, thus leaving between it and the skull-wall above a deep rounded space. 

 A large lateral " sinus " is lodged in this groove, the bone growing round it for two- 

 thirds of its diameter, and, in the case of some birds, viz. the Pigeon, bridging it over 



' This specimen is now in the British Museum ; the bird died too soon after its arrival at the Gardens for 

 specific determination. 



