MITEA. 



121 



wound round the pxllar. The posterior of these plications is 

 nominal y placed between the second and third (counting postero- 

 antenorly) hneations; whilst the anterior is practically conter- 

 minous with the fifth lineation. 



At a later period of the brephic stage, when the normal shell 

 had developed to the extent of a whorl and a half, the whole 

 p.llar became covered with a slight deposit of callus, with three 

 plications the hindermost being decidedly the largest and the 

 anterior the smallest. At two and a half volutions the plications 

 became still more accentuated, though not increased in number 

 But between that period, and until five normal shell-whorls were 

 formed neanic stage), a fourth columellar plication was developed 

 anteriorly. Finally, with the production of seven normal whorls 

 -the most adult example of the species in the Museum-yet 

 another, the fifth, plication was formed. 



From the foregoing it would appear, therefore, that the number 

 of columellar plaits in the brephic stage was only two and the 

 suspicion of a third, and that as the shell increased in size the 

 number of plaits increased also, until in the adult there were 

 no fewer than five of them. Whatever might be the true origin 

 and function of the plaits, we find, in this case at any rate, that 

 they were always developed anteriorly. 



These facts ought effectually to dispose of the view that the 

 number of plications in such a genus as Mitra is per se of specific 

 importance It is quite clear that in many instances the number 

 developed depends on the stage of growth, and, when the animal 

 ot JUdra becomes senile, subsidiary plications are frequently 

 lormed between the normal plaits. 



The ornament of the whorls of M. mxdlhulcata consists, essentially 

 of closely-set small sulcations running spirally, interrupted by 

 fairly equidistant, longitudinal striae or lines of growth, the whole 

 assuming a minutely cancellated aspect. This ornament extends 

 Irora the brephic stage, and persists with extreme regularity to the 

 senile, only that, in the more adult individuals, the growth-lines 

 are somewhat corrugated. The spiral sulcations are multiplied 

 •'s the shell increases in size, additional ones from the protoconch 

 onwards being developed between those already formed The 

 sulcations become much broader on the anterior aspect of the 

 body-whorl The sutures are very slightly cut iu, the whorls 

 oeing a little convex, and not angulate. 



