1879.] Skull and its Nerves in the Green Turtle. 



339 



the first post-oral or tympanic cleft is either suppressed or forms a 

 very slight inner pouch ; it never opens externally in the Amira. 



In most of the Ganoids, as in the Selachians, the first post-oral cleft 

 persists as the spiracle, but in osseous fishes it is a very temporary 

 structure. 



I have long suspected that the archaic (entomocranial) Yertebrata 

 were often supplied with a perfect circlet of branchial filaments 

 around their mouth. 



We know the first post-oral cleft to be a branchial cleft with several 

 branchial filaments ; Dr. Milnes Marshall's researches show that the 

 nasal folds of the embryo Selachian are developed in precisely the 

 same manner as the external branchials of the spiracular opening. 



These filaments become converted into a pseudo-branchia ; the 

 natural suggestion is that the nasal folds, having the same structure, 

 and being formed in a homologous space, are indeed nothing more, 

 even in the adult, than a modified pseudo-branchia. 



Further Remarks on the basi-neural Plates of the Skull. 



I have latterly come back to the same view as to the meaning of 

 the " trabecule cranii," as was first propounded by Rathke, namely, 

 that they are there extensions forward of the basal plates or "invest- 

 ing mass." 



Professor Huxley's term " para-chordal " is as applicable to the 

 hind half of the trabeculae, in embryos of Selachians and Amphibians, 

 as to the pair of plates behind the trabecule, or " para-chordals " 

 proper. 



Moreover, in several kinds of Urodeles, namely, (8]?elerpes, Desmog- 

 nathus, $*c,) there is a hindmost pair of para-chordal cartilages in 

 front of the functionally first vertebra. 



It behoves us, therefore, not to be led astray by mere words ; I 

 should prefer to call the paired cartilages of the notochordal region 

 of the head basi-neural; this term would satisfy not only some of 

 the best authors, such as Groette and Balfour, but it also expresses 

 what I conceive to be the actual nature of the parts. 



These right and left tracts of cartilage that form the main part of 

 the base or skull floor, and grow, more or less, over the overgrown 

 neural axis, are divisible into two regions, the hind region, which is 

 p&m-chordal, and the fore-part, which is prochordal. 



These essentially continuous tracts were, I suppose, developed, first, 

 as a support to the increasing fore end of the neural axis, and from 

 the first were not clearly divided into intercalary (inter-neural) tracts ; 

 and moreover, as I take it, they solidified earlier than the inter-neural 

 tracts of the spinal region. 



That these latter, even, were not always from the first developed 



