48 



PEOF. 0. B. HOWES AND ME. H. H. SWINNEETON ON THE 



exist (cf. PI. V. fig. 12), the quadrate head (#'.) and suprastapedial process (s.st.) 

 being distinct though closely approximate. Passing to Stage Q (figs. 10 & 11), the 

 distance between the two is seen to be much greater, the nearest approximation 



being that of fig. 11. 



Turning now to the suprastapedial process, and the fibrous interval of Huxley. The 

 former arises at Stage Q as a simple upgrowth, seen in section at fig. 10, PI. V., s.st. 

 When Stage K is reached, this is seen to be still elongating (text-fig. 11, s.st.) and to 

 have given rise at the middle of its inner border to a recurrent process (s.st.'), which, 

 passing downwards and inwards, becomes apposed to the columella auris (st.), to 

 which it is tied by a series of fibres, which pass obliquely between the two. Between its 

 recurved lower extremity and the columella a non-hyaline tract is present (*, fig. 12) 



Fig. 11. 



Fig. 11. Drawing from a reconstructional model, showing the detailed characters of the columellar head of 

 SpTienodon and its derivative processes at Stage E, x 30. Fig. 12. Microscopic section through the same, 

 parallel to the columellar axis, x 75. 

 e.c., extrastapedial ; /A, foramen of Huxley ; h'., anterior cornu of hyoid ; ?</., ligamentous fibres ; 



</., upper end of quadrate ; s.st., suprastapedial ; s.st'., recurrent process of the same. 



which we conclude is that described by Huxley. It remains only to add that as the 

 process is absent at Stage Q it is of secondary occurrence. 



As to the continuity between the hyoid cornu and the outer head of the columella 

 (extrastapedial process), we have evidence that for the earliest stages of differentiation 

 (P especially), when all the parts concerned are in the procartilaginous state, it is 

 complete ; but a very interesting detail arises viz., that in some of a series of thick 

 sections which we possess there can be observed (PI. V. fig. 5,*) a very feeble 

 suggestion of jointing, so feeble that it would be likely to be overlooked by anyone 

 not familiar with the sections. This, however, cannot be regarded as evidence of 

 original discontinuity between the columella and the hyoid cornu ! It is seen to be 

 but a passing phase, indicative of either a probable ancestral or an incipient "jointing" 

 such as takes place elsewhere along the cornu (**) ; and, as compared with a later 



