397 



ACANTHOCHITON MAXILLAEIS, 11. 6p. 



PI. xli., figs. 5 and 6; pi. xlii., fig. 1. 



General appearance. — Shell long, rather flat, sides slightly 

 rounded, dorsal area much rounded, width of shell less than 

 half its total length, dorsal area broadly wedge-shaped, all the 

 valves are covered with longitudinal rows of rather large, 

 rounded, mostly porcelain-white pustules, the outer row or 

 rows being irregular in arrangement, all pustules in these 

 being much larger than those in the upper rows and some 

 being twice as long as their neighbours and mammiliform ; 

 here and there there is a tendency for these large pustules, 

 to coalesce. 



Colour. — Shrimp-pink varying in places to geranium-pink 

 (Bidgway's Colour Standards, pi. i.), the girdle is Brussels 

 brown, this colour occurring also in places in the ground-colour 

 of the shell mottled in with the pink. The milk-white or 

 porcelain-white pustules contrast strikingly with the general 

 ground-colour of the shell. 



Anterior valve. — This valve is too broken to disarticulate,, 

 is clothed with white pustules smaller towards the apex, and 

 larger and more rounded towards the girdle ; there are evi- 

 dences of ray ribs, probably five, this feature being so common 

 to Acanthochitons. 



Posterior valve. — Mucro posterior, dorsal area similar to> 

 median valves, broadly wedge-shaped and flat and transversely 

 finely ridged. Balance of valve covered with closely-packed 

 granules, greyish or transparent white, but the granules of 

 the outer row forming the edge of the tegmentum are twice 

 as large as the rest, broad and round, packed closely together, 

 and porcelain- white in appearance. This outer row of pustules 

 gives a scalloped look to the margin of the tegmentum. Inside- 

 white, tinged in places with pink, slits two, the sutural 

 laminae form almost three sides of a square with rounded 

 corners. 



Median valve. — Dorsal area very broad, subcutaneously 

 lined with olive lines, transverse and longitudinal striae, the 

 latter very indistinct. Apex is formed into a broad, rounded, 

 flat beak, which overhangs and is distinctly rugose. The 

 pleural and lateral areas are hardly separable, and there is a, 

 considerable margin of variance between the different valves 

 but all show three or four longitudinal rows of rounded or 

 oval, distinctly separated, milk-white pustules, those next the 

 dorsal area are rather smaller than the lower row, then follows 

 a row of milk-white pustules, fully three times the size of the 

 upper row, and more or less placed alternately, short and long, 

 looking like a row of irregular, rounded teeth set in a jaw, 

 between this row, which rather follows the lines of growth 



