26 



THE VOYAGE OF H.M.S. CHALLENGER. 



It is probable that the Chiton piceus of Angas is this species. He informs us that 

 " this very common species is freely distributed on most parts of the coast of New South 

 Wales. Dwelling in cavities on the upper surfaces of the rocks exposed to the full action 

 of the waves, the valves are generally more or less worn and destitute of character. 

 Length, 1\ inches." 



The two foregoing species have a close external resemblance with each other and with 

 Acanthopleura picea (Gmel.), auct. (non Eeeve). As a matter of fact, the former has 

 frequently been described as Acanthopleura picea in spite of its apparently discontinuous 

 distribution in the West Indies and iu the Indian Ocean. To Mr E. A. Smith of the 

 British Museum belongs the credit of pointing out characters on the under surface of the 

 valves which serve to distinguish these three species from each other. 



These species can be separated on a careful comparison of the sculpture of the shell, 

 but unfortunately the shells are almost invariably so corroded that the surface markings 

 are obliterated. The differences in the character of the girdle are too slight to be of 

 much value ; the light and dark bands of the latter are too variable to be of any use. 



The following analyses refer to two out of many specimens in my own collection of 

 Acanthopleura granulata (Gmel.), from Barbadoes, in future referred to as A. and B., 

 and to a specimen (C.) of Acanthopleura sp. (?) (= Acanthopleura picea, Gmel.) from 

 the West Indies, probably from St Thomas, for which I am indebted to my friend 

 Mr W. H. Dall. Also to the Challenger specimen of Acanthopleura spiniger from Cape 

 York, and to three specimens of Acanthopleura incana. The first two, A. and B., are 

 from Japan, the last (C.) from Port Jackson. Specimen A. was given me by Mr Dall, 

 B. and C. were collected by the Challenger. 



Unfortunately the Challenger Expedition did not obtain any specimens of this genus 

 from the AVest Indies, but as the West aud East Indian forms have been confused with 

 one another, I have not hesitated to deal with them all at considerable length. 



