14 



a broad groove to the tegmentum, colour white tinged with 

 blue. Measures 3 J x 3 J mm. 



Fosterior valve. — Tegmentum very small, about two- 

 fiftlis of total width of valve, anterior portion semi-circular, 

 mucro posterior, dorsal area rather narrow, wedge-shaped, 

 rugose; the wrinkles following the growth-lines are continued 

 across the anterior portion of the dorsal area. The shell 

 behind the mucro is very flat, and shows little sculpture; such 

 as there is, is composed of shallow broken wrinkles following 

 the growth-lines. The dorsal area and the portion behind 

 the mucro creamy-white, side areas dark horn and sculptured 

 with rows of irregular, flat, more or less circular pustules, 

 following the growth-lines and, in places, especially towards 

 the outer margin, coalescing. Insertion plate long and very 

 broad, slit 11, sutural lamina© small and produced forward 

 with a deep inward bent between them and the wings of the 

 insertion plates. This feature is very marked. *A. hednalli 

 has a somewhat similar inward fold at the slits, but this 

 species has this fold on the opposite side of the wing of the 

 insertion plate. Sinus broad, the semi-circular margin of the 

 tegmentum only occupies half the width. Colour of articula- 

 mentum, pale bluish; the slits are continued half-way to the 

 tegmentum in a deep, almost semi-circular groove. This valve 

 measures, longitudinally 3 mm., laterally 4^ mm. 



Median valve. — The following is a description of valve 

 4 : — The dorsal area is smooth except for growth -wrinkles 

 which are continued from the side areas; this area consists of 

 a shallow wedge-shaped depression (this feature is common 

 to all valves except valve 2, in which this area is arched ; a 

 similar variation occurs in a second specimen described here- 

 under), some of the growth-wrinkles are in this area broken, 

 suggesting an ill-defined string of granules. The lateral and 

 pleural areas are not separated but are studded with rows of 

 flat circular pustules, following the growth-lines. The slope 

 of the sides below the elevated fold that margins the dorsal 

 area is slightly curved. Inside, as well as the insertion plates 

 and sutural laminae, pale blue, slits 11, sutural laminae 

 produced forward, sinus narrow and rounded. This valve 

 measures 4 J x 4^ mm. 



Girdle. — Densely covered with long, coarse, cream- 

 coloured spicules; the fringe spicules are a little more slender; 

 sutural tufts long, porcelain-white, and pointed. 



]i{easurement of dried shell, 15 x 6 mm. 



Habitat. — North-west Reef, Barrier Reef, Queensland. 



Remarks. — I am indebted to Dr. John Shirley for the 

 opportunity of examining and describing this Acanthochiton, 



