10 



ACANTHOCHITON (NOTOPLAX) GABRIELI, 11. Sp. 



Differs from A. costatus, Ad. and Ang., in having deep, 

 broken, longitudinal grooving in the dorsal area, whereas A. 

 costatus has smooth, except for the transverse ribs following 

 the lines of growth. 



The pustules in the diagonal ribs of the species under 

 description are larger, more rounded, and irregular than in 

 costatus, and in addition on some of the valves there are 

 evidences of a second coarsely pustulose rib on the posterior 

 margin of the median valves, in this respect approaching its 

 Western Australian ally, A. suh-viridis, Torr. 



Remarks. — The specimen described above has been kindly 

 lent to me for description in this paper by my friend Mr. 

 Gatliff j it is from Caloundra, in Queensland, and the type, 

 which has not been disarticulated, remains in Mr. Gatliff 's 

 collection. I am naming it after my friend Mr. Charles J. 

 Gabriel, who has for so long been associated with Mr. Gatliff 

 in the excellent conchological papers they have jointly pro- 

 duced. While I suspect this form should hold sub-specific 

 rank only, it is as much entitled to the higher rank as are 

 some of its allies referred to in the discussion following. 



AcANTHOCHiTON COSTATUS, Ad. and Aug., and its allies. 

 (Ad. and Ang., P.Z.S., 1864, p. 194; Aiigas, I.e., 1867, p. 224.) 



The examination of the preceding species from Caloundra 

 and, subsequently, the loan of a specimen from Port Philip, 

 Victoria, measuring 7i mm. in length, by Mr. Gabriel, has 

 made it necessary to go into the whole question of the respective 

 relationships of several nearly-related species. 



My series of A. costatus include three from Sydney, 6, 

 12, and 19 mm., respectively; several from Tasmania, up to 

 36 mm. in length; and one from South Australia, 21 mm. 

 All have minute, slender, girdle spicules, easily detached ; all 

 have several ribs composed of coarse pustules, behind the 

 mucro, in tail valve, but in the smallest this feature is repre- 

 sented only by a single large pustule at the outer edge ; all 

 probably^ in the quite juvenile stage, possess none of these 

 posterior ribs. In the larger specimens the original form of 

 the dorsal area in the juvenile is not quite clear, but in the 

 three from Sydney Harbour, 6, 12, and 19 mm. respectively 

 (dry), it certainly commences with a prominent, broad, 

 rounded beak, the area rapidly widening with sundry jags at 

 each side, giving the pinnatifid character. On reaching a total 

 length of the whole shell of 6 to 10 mm., the dorsal area may 

 slightly contract or continue in two parallel lines, forming in 

 the adult a narrow raised dorsal ridge. On collecting the 



