BY CHARLES HKDLEY. 415 



In tropical latitudes the family Onchidiidaj are usually numer- 

 ous; their complete absence at Funafuti therefore struck me as 

 remarkable. Samoa and Tonga appear to be their farthest 

 stations in the West Pacific. Their limited powers of migration 

 are explained by the statement of J. Joyeux-Laftuie that — "The 

 whole development of Onchidium takes place within the egg, and 

 the young at the time of hatching already possess the form of the 

 adult."* 



I should suppose indeed that the bulk of the molluscan fauna 

 reached Funafuti in the larval swimming stage. 



Fischer records! having taken in the open sea a Triforls which, 

 although eight or nine whorls of the shell were formed, still 

 retained the larval, i.e., swimming characters. In the light of 

 this statement the wide range which Trifo7'is enjoys in Polynesia 

 might have been anticipated. 



Little is known of the comparative endurance of the swimming' 

 larval stages of Mollusca. That Pelecypoda range farther than 

 GasterojDoda suggests that they swim longer. Trifonium and 

 Cerithiujn should by their distribution be gifted with unusual 

 swimming powers. 



That the Polyplacophera sliould only be represented l)y a frag- 

 ment in the roll of the Funafuti Mollusca is quite in keeping 

 with the distribution of this order in the Central Pacific. But 

 six species were known from this region to Harper Pease, who in 

 his last paper wrote — "The absence of Chitonidie from Polynesia 

 has been noticed by authors as a remarkable fact, abounding as 

 they do in the surrounding provinces, especially on the west coast 

 of America, at Australia and New Zealand. "| 



Interesting re-iults would be reached by tabulating marine 

 Invertebrata, according as they travel much, a little, or not at 

 all, in the early stages of their development; by plotting the 

 geographical distribution of each, and comparing the results. 



* Joyeux-LafiFuie— Archives Zool. Exper. x. 1SS2, p. 3;i8. 

 t Fischer — Manuel Conch., 1SS7, p. 679. 

 J Pease — Am. Journ. Conch, vii. 1872, p. 194. 



