THE SKELETON OF THE VERTEBRATE HEAD. 121 



of tlie palate, alveolar margin, and floor of the nasal fossaj. 

 The only trace of ascending processes which they present is a 

 compressed spine, which projects upwards at their junction in 

 the median line of the nasal opening of the cranium. Tlie 

 lateral margins of that opening are formed by the ma.xillaries 

 alone; ; its upper margin by the so-called pre-froutals, except 

 in Ilydromedusa and certain fossil forms. The cartilaginous 

 septum of the nasal fossie extends up from the intermaxillary 

 suture to that of the pre-frontaLs. 



Is the chelonian vomerine sclerotome modelled on that of 

 the crocodile, or of the bird ? The chelonian presents the first 

 stage in the remarkable development of the nasal passages 

 exhibited by the crocodiles. But the general deficiency of the 

 nasal bones, the indications of ascending processes of the 

 intermaxillaries in the mesial plane, the formation of the 

 posterior margins of the external nostrils by the maxillarics, 

 appear to me to show that in the construction of its vomerine 

 sclerotome, the chelonian differs from the crocodiles, and 

 resembles the typical lacertians, the ophidians, amphibians, 

 and the birds. The cartilaginous lining of its nasal fossa% a 

 remnant of its primordial cranium, projects, in general, a little 

 beyond the margins of the osseous nostrils, as in birds ; but 

 in Trionyx and Chelys, the projecting margins run forward 

 together in the form of a double cartilaginous proboscis. 



Hic Vomerine Sclerotome in tlie Osscoi's Fv<hcs. — I have 

 already described the general constitution of the vomerine 

 sclerotome of the osseous fish, as one form of deterioration of 

 the fore part of the cranium. Its centrum, the vomer, wlien it 

 is present, is merely a cartilaginous nodide in the longitu- 

 dinal axis of the basis of the cranium, in front of the bone 

 usually described as the " vomer," but which I believe to be 

 the centrum of the ethmoidal sclerotome ; the neural elements 

 are those scale-like bones, which Cuvier recognises, I believe 

 con-ectly, as the nasals. Professor Owen considei-s these 



