105 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



PLAXirHORA, Gray, 1847. 



Key to Species. 



a. Valves exposed. 



b. Posterior valve not greatly reduced in size or altered in form. 

 c. Central areas unsculptured save for growth-lines. 

 d. Sutural pores or tufts distinctly developed. 



e. Lateral areas with subobsolete radiating riblets. supcrha. 



ee. Lateral areas with at least two distinct radiate ribs. mhatrata. 



dd. Sutural pores absent, girdle densely covered with bristles. Sutcri. 

 cc. Central areas sculptured, at least at the sides. 



d. Large. Sutural pores with bifurcating bristles ; girdle 



broad, reddish. biramosa. 



dd. Small. Sutural pores with more than two bristles; girdle 



narrow, white or white aud black. ctclata. 



hh. Posterior valve reducid to a narrow crescentic form, strongly 



arched upward. ovata. 



aa. Valves partially immersed in the girdle, which encroaches at the 



sutures. obtccta. 



7. Plaxiphora. bieamosa (Quoy & Gaimard). 



Chiton biramosits, Quoy & Gaim. : Voy. Astrolabe, Zool. vol. iii 



(1835), p. 378. t. Ixxiv, figs 12-16. 

 Acanthochaies liramosus, Quoy & Gaim. : Huttou, Trans. New Zealand 



Inst., vol. iv, p. 181. 

 Plaxiphora liramosa, Quoy & Gaim. : Hutton, Man. New Zealand 



Moll., p. 116 ; Pilsbry, Man. Conch., ser. I, vol. xiv, 



p. 319, pi. Ixviii, figs. 51-4. 



Young specimens I always found to be sculptured like P. coilata, 

 only more grotesquely, but in adult specimens tbe sculpturing is 

 generally more or less effaced. Sometimes there is a beautiful ornamen- 

 tation with green, pink, white, and red-brown, but as a rule the valves 

 ai'e covered with coralline growth and seaweeds, which render it very 

 difficult to distinguish the animals from their surroundings. The 

 colour of the girdle varies very much Avith age ; in young specimens 

 it is sometimes of a dirty orange, whilst in adult forms it is dark red- 

 brown. A feature that has hitherto been overlooked is the notch at 

 the posterior end of the girdle ; this, however, is not always very 

 distinct. In some young shells I found it to be deep and broad, but in 

 adult specimens mostly reduced to a more or less deep slit. I have 

 met with a specimen measuring 60 by 40 mm., divergence 150°. 



Hah. — Wellington (Otago Museum) ; near Lyttelton (H. S.). 



P. biramosa is one of the very rare New Zealand species. The 

 specimen in the Otago Museum from Cauipbell Island, labelled 

 P. biramosa, is not that species, but P. suhatrata. 



8. Plaxiphoea superba, Pilsbry. 



Plaxiphora supcrha, Cpr. MS. : Pilsbry, Man. Conch., ser. I, vol. xiv 

 (1893), p. 319, pi. Ixviii, figs. 55-61. 



