1888.] Structures found in the Skull of Birds. 



399 



pared with various figures in my Memoirs on the Mammalian Skull 

 (Parts 1, 2, and 3, 'Phil. Trans.'), it will be seen that it so nearly 

 corresponds with sections of the skull of the Pig, the Edentata, and the 

 Insectivores, especially those taken just behind Jacobson's organ, that 

 without explanation it would be impossible to tell which figure 

 belonged to the Bird, and which to the Mammal. 



In the Fowl, the Duck, and other Precocial birds, the embryo of the 

 eighth day of incubation is in the Amphibian stage ; then, the web 

 goes beyond the toes, on the foot, whilst the rudimentary wing shows 

 clearly a paddle with three digits in it, the first shortest, and the 

 third not much shorter than the second. Very soon, however, after 

 this, the first and second digits acquire a claw. Thus the Reptilian 

 stage has been attained ; for I know of no existing Amphibian with 

 claws, except the Cape Nailed Toad (Dactylethra) . 



Before I had ever offered any of my Morphological Papers to the 

 Royal Society, I had stumbled upon a part in the development of the 

 skull in the chick, the importance of which is to me much greater now 

 than ever. 



Since that time, besides working out the development of the skull in 

 many types of Birds, all the Ratitae except the Apteryx, and in family 

 after family of the Carinatse, I have had the opportunity to work it out 

 in all the main types of existing Reptiles ; the result is to me very 

 remarkable. 



In all the Ichthyopsida, except the cartilaginous Fishes — Marsipo- 

 branchs and Elasmobranchs (Hag, Lamprey, Shark, Skate), the base 

 of the skull is supported by a long splint-bone, the nature of which 

 had been completely misunderstood by the old school of anatomists, 

 but which was put into its right category by our great Reformer, 

 Professor Huxley, and called by him " parasphenoid." 



This great superficial basi-cranial beam is largest in those Ganoids 

 that are half Selachian — the Sturgeons and their allies : but it is 

 very large in the other (Holostean) Ganoids, in the Teleostei, Dipnoi, 

 and all the Amphibians. 



It is not part of the true skull, it is the subcutaneous part of a 

 dermal scute, formed inside the infolded skin of the mouth, and is a 

 truly Teleological bone, developed for support to badly ossified endo- 

 crania, just as such skulls are supplemented by dermal bones — " Der- 

 mostoses " or " Parostoses," above, and on each side. This bone, well 

 seen in the Frog, is dagger-shaped, and reaches from near the foramen 

 magnum behind, to the nasal capsule in front, the " guard " of the 

 dagger supporting the auditory capsules. Now in Serpents only the 

 blade is present ; in Lizards only a very fine thread of bone repre- 

 senting the blade; in some, e.g., Trachydosaurus rugostis (" Cyclo- 

 clontidee "), even this is wanting. It is not present in those very 

 Amphibian forms the Chelonians ; and in Crocodiles, I can only find 



