82 MR. W. K. PARKER ON THE STRUCTURE AND 



Tlie skull of the Tadpole, evidently a most generalised structure, covered the whole 

 question with a cloud of doubt. 



Since then, whilst wavering, I have collected copious evidence of what seems to me 

 to be a true second pre-oral rudiment in the antorbital region, and a first or terminal 

 rudiment in the front of the face. 



I have already shown (" Batrachia, ' Part II., and " Urodeles,"' Part I., and in my 

 papers on the " Selachian and Avian Skulls ") the very constant occurrence of an 

 ethmo-palatine in several large natural groups of Vertebrata. 



Here, in the larva of Pseudis, 1 have shown this part jjartly segmented off, and 

 when not segmented off, it is yet very distinct, and not to be misunderstood; it assumes, 

 in fact, four positions, each of whicli is Uke what is seen in the members, generally, of 

 some large group or groups. 



This large and most I'emarkable conjugational bar is parallel with the skull in the 

 two first stages (A and B, Plates 2 and 11). 



So it is in the Siluroid Fishes {e.g., Clarias capensis), where the palatine bone 

 formed by the ossification of an autogenous cartilage (in these and other Teleostei) 

 I'emains distuict, but has no " ethmo-palatine" or ascending process, as in the Salmon. 



In the third stage (C) it is bent outwards and forwards ; this is the natural form 

 and position of its independent homologue in a large number of the " Urodeles." 



It afterwards turns round ; but in douig this there is a time in which it is du'ectly 

 transverse, or at a right angle to the skull, as in Menohranchus. 



In Proteus anguinus, where it is not drawn forwards by a chondrified nasal capsule, it 

 turns backwards, and so it does also in Notidanus, in the Skates generally, and in 

 many Birds, 



In the Salmon, as in the adult Toad, the ethmo-palatine has its three regions well 

 developed — an ascending, an anterior, and a posterior part ; all these are well seen in 

 the stage of Pseudis just described. 



Such a rudunent of the ethmo-palatine as exists in the adult of many Urodeles, 

 where it lias coalesced with the back and lower part of the nasal capsule, is seen also 

 in the "Sauropsida" {e.g., Chamwleo, Drornoeus, Casuarius, and Struthio); whilst in many 

 Birds it is separately ossified, and forms, as the " os uncinatum," a most characteristic 

 endoskeletal bone in the fore-palate. 



The foremost pleural rudiment is less widely distributed as the " j^ro-rhinal," or 

 "recurrent trabecular cornu." I should not be surprised if it turned <)ut to be the 

 serial homologue of the mandibular suspensorium, of the epi-hyal, and of the epi- 

 branchial elements. ■■'" 



* I liavc already mcnliuucd (p. 18) that Professor lIuxMiY aud Mr. Ealfouk do uot take the same views 

 ul these parts as Professor Milnes Mausuall aud I do. 



