89 



Definition of a New Species of South 

 Australian Polyplacophora. 



By M. M. Maughan. 



[Read September 3, 1900.] 

 Chiton OPUktUS, sp. nov. PI. i., figs. 3a— g. 



General Appearance. — Shape oblong, oval, very elevated, 

 carinated, side slopes steep and slightly curved. Shell sloping 

 back from about valve five. Colour. — Greyish green, mottled 

 with olive. Anterior valve slightly lighter in colour than the 

 rest. Dorsal areas tipped with reddish tinge. Girdle same 

 colour as valves, with dark olive stripe opposite each lateral area, 

 and at intervals round anterior and posterior valves. 



Anterior Valve. — Steep, almost smooth, but showing scarcely 

 perceptible radial costse, and under the lense concentric wrinkles, 

 faintly imbricating. Slits eight. 



Posterior Valve. — Small, about half width of valve three. Mucro 

 anterior. Posterior slope nearly straight. Posterior area orna- 

 mented with faintly imbricating concentric wrinkles like the 

 lateral areas of other valves. Immediately in front of this area 

 are the deep wedge-shaped pits described below. Slits eight. 



Median Valves. — Dorsal area beaked, merging into the pleural 

 area, faintly wrinkled w 7 ith what appear to be growth lines. The 

 marked feature of this shell is the row of about 12 parallel, very 

 short, deep, wedge-shaped pits in the pleural areas, beginning 

 from the edge of the lateral areas. They make an angle of about 

 120° with the direction of the transverse growth lines. Lateral 

 areas are elevated with very low, flat, longitudinal, faintly 

 imbricating divisions. Slits 1.1. Insertion plates with pectinated 

 sinus broad, denticulated with 12 slits. A strong triangular rib, 

 with apex at the posterior margin of the insertion plate, is seen 

 inside of each of valves two to seven. Colour of articulamentum 

 greenish white. 



Girdle. — Scales smooth, wide, and very obtuse at apex. 



Measurement. — Length, 20 mm.; width, 10 mm. 



Habitat — McDonnell Bay ; found by Dr. W. G. Torr, to 

 whom 1 am indebted for specimens. 



EXPLANATION OF PLATE I. 



a. Dorsal view of entire shell. 



b. Anterior valve. 



c. Median valve. 



d. Posterior valve. 



e. Anterior valve showing teeth. 

 /. Portion of girdle magnified. 



g. Insertion plate, median valve. 

 Fi£. 



1. Ischnochiton (Stenochiton) pallens. 



2. Callochiton rufus. 



3. Chiton oruktus. 



