MEGACEPHALUS AND P. BRACHYCEPIIAHJS. 



475 



It must be borne in mind, however, though. I doubt whether it is 

 generally known, that the posterior nares by no means occupy 

 a constant position in the Lacertilia ; for though they are most 

 usually bounded in the manner above stated, they sometimes shift 

 their position backwards and open at the back of the palatines. In 

 such cases, however, the palatines are produced towards the middle 

 line, each along its outer edge, into an underlying plate, which roofs 

 over the mouth and forms a floor to the nasal passages. A section 

 across the Plesiosaurian skull might be expected, therefore, to give 

 some signs of an inflection of the palatine bones, converting them 

 into incomplete tubes, if such a backward extension of the nasal 

 passage obtained in it. INTo such signs, however, are to be detected 

 in the specimens under consideration. A diagrammatic sketch of a 

 fracture passing transversely through is given below (fig. 12) ; it 

 shows plainly the outward and upward bend of the palatines, but 

 not a trace of an infolding. 



Fig. 12. — Transverse Section across the Shall of P. megaeephalus, 

 showing the piloted plates flanged upwards and outwards towards 

 the Maxillos^ hut not inflected to form a nasal passage. 



It is a fact too curious to be passed over, however, that the 

 internal are situated in advance of the external nares of this 

 Plesiosaur, the anterior margin of the latter being a trifle under 

 2*8 inches behind that of the former. 



A transverse section through the upper and lower jaws is given 

 in fig. 13 : the upward flange of the palatine is seen meeting the 

 maxilla ; and the lower jaw has the usual reptilian composition. 



2. Redetermination of the Number and Distribution of the Ver- 

 tebrae and of the Length of the Regions of the Spinal Column. — Of 

 cervical vertebrae, twenty-nine are visible up to the anterior edge 

 of the furculum; in all probability one more lies beneath this 

 bone ; and the total number may therefore be taken as thirty. 

 In consequence of the concealment of a large part of the spine 

 beneath the pectoral and pelvic girdles, the number of dorsal 



