IREDALE : THE CniTONS OF THE KERMADEC ISLANDS. 41 



of growth. This implies tliat the primitive form was unsculptured, 

 and the sculptured forms are more recent. 



It is most interesting from this point of view to study the Austra- 

 lasian Rhyssoplax when we find this primitive form surviving 

 unchanged in the species Chiton translucens, Hedley & Hull. The 

 next stage is well known by means of Ch.jugosus, Gould, Ch. coxi, 

 Pilsbrv, etc., and the third stage by such species as the succeeding 

 one and Ch. <Brem, Reeve. A further development of stronger and 

 more pronounced sculpture still is seen in Ch. canalictdatm, Quoy and 

 Gaimard, and C. vauclui^ensis, Hedley & Hull. A still more com- 

 plicated stage is exemplified by Ch. Iwians, Pilsbry, where, in addition 

 to the production of strong sculpture, the giidle- scales develop, from 

 ordinary convex scales, into abnormal mucronate ones. I have traced 

 this species through the stages noted. An extraordinary and different 

 mode of procedure is that adopted by Ch. hotvensis, Hedley & Hull. 

 This species commences as a normal unsculptured shell, but no pleural 

 sculpture is formed, and, instead of radial ribbing on the end valves 

 and lateral areas of the median valves, concentric ridges are produced. 

 The onlv other species yet known to be equally aberrant is Ch.platei, 

 Thiele (Revision, p. 92, pi. ix, figs. 46-8, 1909), described from the 

 Red Sea, whose radula Thiele has shown to be normal to this group. 



Rhyssoplax exasperata, n.sp. PI. II, Fig. 13. 



Shell of medium size, broadly elongate oval, elevated, not definitely 

 keeled, side slopes nearly straight, girdle scaly. Colour variable, 

 green splashed with lighter or darker being the predominant tints; 

 the green may be very pale or dark ; white prevails in a few s[)ecimens, 

 but no absolutely uniformly coloured shell was obtained, though 

 practically a white one and a black-brown one were noted. Anterior 

 valve rayed with twenty raised ribs, slightly nodulous; at the outer 

 edge intercalating riblets occur in adult specimens. Median valves 

 have their lateral areas similarly four- or five-ribbed ; the pleural areas 

 are sculptured with slanting very closely packed sulci, twelve or more 

 in number, none of which reach the anterior edge of the valve, and 

 vanish before the dorsal area is reached, thus leaving the jugal tract 

 smooth and polished. Posterior valve has the mucro elevated, before 

 the centre, the anterior portion sculptured as the pleural areas of the 

 median valves, the posterior portion as the anterior valve, the ribs 

 being fifteen or sixteen and more nodulous in character. Inside 

 coloration greeni.sh, but varying a little, according to the external 

 coloration. Anterior valve has a slightly projecting insertion-plate 

 regularly cut by eight slits, the teeth beautifully pectinated. Median 

 valves with the insertion-plate one-slit, the sutural laminae low and 

 broad, the sinus nairow and finely denticulate. Posterior plate less 

 projecting than anterior, but more developed at sides than centre ; the 

 slits number eleven, but one is disproportionate, whilst the others are 

 fairlj- equal, thereby suj^gesting twelve to be the normal number. 

 Girdle covered with small oval scales, very closely imbricating, and 

 regularly finely grooved. 



