BY H. LEIGHTON KESTRVEN. 



329 



Cymatium, and probably grew, pari passu, with the calcareous 

 ananeanoconch, exactly as does the adult periostracum and shell; 

 moreover, in Megalatractus the horny covering is extremely thin. 

 All the last three stages may be shown to be present or absent 

 by the study of one or two growth-stages of the shell, and by 

 taking into consideration the size and sculpture of the protoconch. 

 Taking Megalatractus aruanus Linn., as being a good example 

 wherewithal to illustrate the deductive methods which may be 

 brought to hear on the subject.^" From the large size of the 

 mollusc when it escapes from the egg-capsule, we md^y deduce 

 the fact that the velum had been aborted some time prior to that 

 date; from the fact that the whorls are angled and nodulose, and 



Fig. 1. Fig:. 2. 



Fig. 1. — Protoconch (nat. size). Fig. 2. — Apical whorls of decollated 

 protoconch showing the variation in the direction of its long axis and 

 apparently in size, of the portion lost. 



that some are striate, we deduce a mantle assuming the adult 

 condition, and therefore well developed; this we ma}'- the more 

 safely do from the fact that there is no demarcation between the 

 later whorls of the protoconch and the earlier whorls of the 

 conch, in short no signs of nepionic interruption. Again, there 

 is a smooth thimble-shaped nuclear portion very distinctly marked 

 off from the coiled whorls. The conclusions that may be arrived 

 at from these deductions are that the smooth nuclear portion is 



* A full description of the protoconch is given in my report on the 

 "Anatomy of Megalatractus aruanus Linn." (5). For the opportunity of 

 reproducing the figures, I have to thank the Trustees of the Australian 

 Museum, who kindly granted the loan of the text-blocks. 



