178 ME. W. K. PARKER ON THE STRUCTURE AND 



form. I would now add to this category the marginal plates of the cranial bucklers of 

 the recent and extinct ganoid Fish, viz., Sturio, Dipterus, Osteolepis. The bone in the 

 Sturgeon, which Professor Huxley has compared to the so-called squamosal (pterotic) 

 of the osseous fish, is merely one of this category ; so also are those in front of it, one 

 of these being the homologue of the postorbital bone of the Reptilia, and a mere splint- 

 bone, and not the homologue of the postfrontal of the Fish and the Bird. I am aware 

 that in the Skinks and Blind-worms the process of ossification has affected much more 

 of the derm-fibres than in the bird, only a thin layer being left as a quick for the epi- 

 dermic cells, which form the investing scales ; this, however, does not alter the homology 

 of the parts ; nor would it if the whole skin were converted into hard, naked, enamelled 

 bone. Any one familiar with the histological development of these parts will make no 

 difficulty here. 



The mandibles of the Tinamou (Zool. Trans, vol. v. pi. 40, figs. 3, 6, 7) are as truly 

 struthious ; the thoroughly cemented symphysis is 5^ lines in extent in T. variegatus, 

 and the lateral lines and the large puncta; are the precise counterpart of those in the 

 upper jaw. The distinctness of the mandibular splints is feebly shown, and there is no 

 open space ; the angles of the jaws (external and internal) are but little produced ; the 

 latter, the most important, being the homologue of the " manubrium mallei," is in a 

 low developmental condition, this part being relatively largest in the little " hammer " 

 of the Mammal, and completely wanting in the large "articulare " of the osseous Fish. 

 They are both large in the Gallo-anserine series of birds; and the internal angular 

 process is large in Finches and Crows, 



The OS hyoides (Plate XV. fig. 12) is much like that of the Gallinacese in outward 

 form, but is in reality much less developed ; the narrow cerato-hyal cartilages [c.h.) have 

 almost coalesced ; they are distinct at the middle, and they diverge behind. The basi- 

 hyal {b.h.) is not distinct from the uro-hyal {u.h.) ; but a short bone is developed in the 

 proximal part of the latter ; it is rounded, and blunt at the end. The lowest thyro-hyal 

 {th. 1) is almost entirely ossified, and the upper [th. 2) has only the tip cartilaginous ; 

 these parts come very near to those of the Fowls, but they have a bony basi-hyal also ; 

 and in them the posterior two-thirds of the partly coalesced cerato-hyals has become 

 bony. 



