Plaxiplwra.} AMPHINEURA. 25 



Length, 15 mm. ; breadth, 13 mm. Divergence, 112. 



Type in the Dominion Museum, Wellington. 



Hob. South Island and Wellington Harbour ; mostly found in 

 kelp-roots. 



Remarks. The black spots on the valves are not always present. 

 A specimen with 6 valves only is recorded by Iredale (T. N.Z.I., xl, 

 375, pi. 31, f. 1). 



Fam. ACANTHOCHITID^J, Pilsbry. 



Chitons in which the valves are more or less immersed in the smooth 

 or hairy (never scaly) girdle ; the tegmentum, or outer layer, therefore 

 being much smaller than the articulamentum, or inner layer, and 

 having the exposed surface (when present) divided into dorsal (or 

 jugal) and latero-pleural areas, the latter formed by the union of 

 lateral areas with the sides of the central areas. Insertion teeth sharp, 

 nearly smooth. Body not vermiform. Posterior valve either slit 

 similarly to the head valve or having a posterior median sinus ; the 

 mucro submedian. 



Besides the positive and negative characters given above, the 

 species of this family generally have 5 slits in the head valve, and 

 median or short gills. (Pilsbry.) 



Genus 1. ACANTHOCHITES, Risso, 1826. 



Acanthochites, Risso, Hist. Nat. cle FEurope Merid., iv, 1826, 2(38. First 

 species : A. communis, Risso. 



Valves partially buried in or covered by the girdle, the exposed 

 part consisting of a smooth or striated dorsal band, and granulated 

 side areas, the latter sometimes lacking. Anterior valve with 5 sym- 

 metrically placed slits ; median valves with 1 slit on each side ; pos- 

 terior valve with 2 or several slits. Girdle varying from densely hairy 

 to naked, but always having 4 bristle-bearing pores around the head 

 valve, and a single series of pores on each side placed at the sutures. 

 Gills short, extending forward from one-third to three-fourths the 

 length of the foot. (Pilsbry.) 



The girdle-pores seem to be a constant generic character ; they are 

 normally 18 in number. 



Distribution. World- wide. 



KEY TO SPECIES. 



a. Anterior valve without radiating ribs ; not obviously lobed 



around the lower edge of tegmentum. 

 b. Tail valve with one slit on each side ; girdle covered with 



spicules and having well-developed tufts . . . . zdandicus. 



bb. Tail valve with several slits ; girdle naked, leathery, cover- 

 ing the valves except for a linear band at the ridge ; 

 small tufts on tubercles . . . . . . . . porosus. 



