586 MR. J. B. SUTTON ON THE DEVELOPMENT [Juue 2, 



passes towards the anterior limit of the fronto-nasal plate. Later the 

 rod becomes segmented as follows : — the distal end becomes internal 

 pterygoid plate, the middle portion persists as the cartilaginous 

 piece of the Eustachian tube, and the proximal portion degenerates 

 into ligament. Tlie details of the metamorphosis of this bar will be 

 found in my paper on the " Paraspheuoids " &c. (Proc. Zool. Soc. 

 November 1884). 



In the Crocodile the corresponding bar of cartilage is continuous 

 posteriorly with tlie huge quadrate, its middle portion is considerably 

 infringed by the so-called pterygoids, whilst its distal end is conti- 

 nuous with the OS trans versum. 



If the bones forming the posterior limit of the hard palate in 

 Crocodiles are not to be regarded as pterygoids, to what do they 

 correspond? They are the homologues of the avian basitemporals. 

 As is the case with the so-called pterj-goids of the Crocodile, the 

 bird's basitemporals are preformed in membrane, they underlie the 

 basisphenoid, and the Eustachian tubes lie above them ; but the bird 

 in its aerial mode of life needs not a long tubular nasal passage, 

 indeed its hard palate may bs considered defective, and each basi- 

 temporal, instead of sending a process of bone to curve around the 

 posterior nares, merely persist as a flat plate of bone which even- 

 tually becomes welded to the skull-base. 



In birds the pterygoids take a different direction from that of the 

 Crocodile's os transversum : in the former case they converge ante- 

 riorlj', and in some avian skulls actually come into contact at the spot 

 where they join the palatines, whilst posteriorly they abut upon the 

 quadrate bone ; this last fact is sufficient to prevent any misinterpre- 

 tation as to their nature. In the Crocodile the anterior ends of the 

 pterygoids are carried outwards, until they rest on the maxillae, and 

 the postpalatine bones (the so-called pterygoids), being wedged in- 

 between them, separate the pterygoids from the quadrate to such 

 an extent as to disguise their real nature and make them appear as 

 additional ossifications. 



According to this view the pterygoid of Birds, the os transversum 

 of Crocodiles, the transpalatine of the Snake, and the internal 

 pterygoid of Mammals, including iNIan, arise in connection with the 

 distal end of the palato-quadrate cartilage, and must therefore be 

 regarded as homologous bones. 



The so-called pterygoids of Snakes and of Crocodiles and the basi- 

 temporals of Birds agree in their mode of development and relation- 

 ship to the main morphological landmarks of the skull; they must 

 therefore be regarded as homologous ossifications, and as a matter of 

 convenience it is proposed to name them postpalatines. Whether 

 these ossifications are represented in the mammalian skull by the so- 

 called sphenoidal tubinals, or by certain accessory ossicles which are 

 developed in connection with the hinder end of the vomer in some 

 types (marsupials, hedgehog, &c.), or not represented at all, is a matter 

 of very little importance. It would of course be very interesting to 

 be able to determine whether the bones which prolong the hard 

 palate in some of the Edentates, Myrmecophaga for example, arise 

 in the same manner as the bird's basitemporals. My conviction, so far 



