396 



and broken into incipient, flattened pustules. Insertion plates, 

 porcelain white, slits five, broad. 



Posterior valve. — Mucro very distinct, posterior, the 

 anterior portion of valve is similar in sculpture to the pleural 

 and lateral areas in other valves. A diagonal depression 

 separates this from the posterior portion, the ribbing being 

 deflected downwards and its character somewhat altered, the 

 ribs here showing a tendency to become granulose, still further 

 changing when the posterior part of valve is reached, the shell 

 there being covered with closely-packed granules without any 

 system of arrangement. Insertion plates white, one broad slit 

 on each side and four, and suggestion of a fifth, immediately 

 behind the mucro. 



Median valves. — The dorsal area is longitudinally lined 

 with whitish lines separated from one another by darker lines 

 which look like grooves, but under a stereoscopic microscope 

 the surface is found to be practically ungrooved longitudinally, 

 but crossed by shallow transverse sulci. Strictly speaking 

 there is some evidence of shallow longitudinal grooving exist- 

 ing in places ; this feature may be more marked in other 

 specimens. 



The pleural area is covered with close, wavy, longitudinal 

 ribbing, the ribs are more abrupt on the lower side, and the 

 trough between them is broad and shallow ; both ribs and 

 trough are diagonally scratched or minutely grooved. The 

 lateral area is sculptured in a similar manner to the pleural, 

 but the ridges are deflected upwards on reaching the diagonal 

 undulation, it can barely be called a rib, which separates the 

 two areas ; the lateral area is very small compared with the 

 pleural. Inside of shell white, median valve one slit, sutural 

 laminae produced very little forward, sinus broad and sinuate. 



Measurements. — 35 mm x 11 mm. in dried specimen. 



I am indebted to Dr. W. G. Torr for the opportunity of 

 describing this very fine Acanthochiton ; it was dredged in 

 Gulf St. Vincent, South Australia. Up to the present only 

 one specimen has been met with. The type will remain for the 

 present in Dr. Torr's collection, but ultimately it will be placed 

 in the South Australian Museum. 



Remark*. — This species can easily be distinguished from 

 Sotoylax matthewsi, Bed. and Pils., by the ribbing being 

 continuous and not broken into granules, the ridges are less 

 strong, and the pinnatifid character of the dorsal area, so 

 marked in N. matthewsi, is almost absent in this species. It 

 is more nearly allied to that species than to any other Acantho- 

 chiton known to me. The specific name is derived from the 

 Latin porca, meaning a ridge between furrows. 



