ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 307 



any break into the median dorsal fold of the oesophageal mucosa. The velum may thus 

 be, and actually is, in one sense, merely the differentiated anterior extremity of the 

 median dorsal fold of the oesophagus which has developed an elaborate skeletal support. 

 On the other hand, the velum may represent the original roof of the gut cut off by 

 lateral evaginations (cp. fig. 11). This agrees rather with the anatomical facts, and 

 with what we know of the development of this structure. 



The lateral edges of the velum are curled over sharply dorsally, and its posterior 

 edge, when spread out, exhibits a short, blunt, median projection stiffened by the posterior 

 transverse velar bar with its irregular posterior processes, and also on each side a 

 pointed projection, the inner edge of which is supported by the termination of the 

 internal lateral velar bar, and the outer or turned-over edge is strengthened by the 

 external lateral velar bar. The ventral or flattened portion of the velum is sustained 

 by the internal lateral velar bars and their anastomoses, whilst the dorsal doubled-over 

 portions are stiffened at their edges by the external lateral velar bars. The median 

 suspensory portion of the velum, which is more definitel}^ developed behind than in 

 front, is supported by the supra-pharyngeal skeleton associated with the anterior 

 transverse velar bar (cp. Parts I., II., and III.). 



The mucosa covering the velum is smooth. 



If the roof of the mouth be examined, we find the mucosa thrown into coarse folds 

 — probably due at least partly to contraction, since with preservatives such as Perenyi's 

 fluid they are less obvious. Laterally in front are the conical third and the obtuse 

 fourth tentacles. Just behind the latter in the mid-dorsal line will be found a pit, and 

 rising from the bottom of this pit is an elevation of the mucosa, from the apex of which 

 emerges the strongly curved, slender, median dorsal tooth. Immediately behind the 

 level of the posterior margin of the dental apparatus (in the retracted condition) is a 

 large deep recess, the ventral wall of which is formed by the mucosa of the roof of the 

 mouth, which apparently terminates here in a somewhat irregular border, as figured by 

 W. K. Parker (PI. XIII., fig. 7), and the dorsal wall of which is formed by the base 

 of the velum. This is the recess into which the naso-pharyngeal canal opens, and the 

 latter aperture is seen if the free border above is drawn forwards. It is an elongated 

 oval opening, wider in front than behind, and with an indefinite posterior border, 

 owing to its being prolonged on to the base of the velum as a furrow, on which it is 

 continued almost as far backwards as the anterior transverse velar bar. The mucosa, 

 in fact, is doubled to form the border mentioned above, then becomes continuous with 

 the mucous lining of the naso-pharyngeal canal at the posterior opening of the latter, 

 is next continued on to the velum, and finally passes into the mucosa of the roof of the 

 mouth and oesophagus behind the velum. On each side of the base of the velum 

 dorsally and laterally is a deep, forwardly projecting pit with an almost smooth lining. 

 Into the blind end of this pit a fold of mucosa projects in front. Sections, however, 

 show this to be due to the root of the external lateral velar bar. The pit probably 

 disappears when the dental apparatus is everted, and perhaps, therefore, only exists 



