154 Edwin Ashby : 



minutely punctate ; central areas having about 14 grooves on each 

 side, obsolete in the middle ; lateral areas having two riblets,. 

 sometimes bifurcating, or with another itercalated, furnished with 

 strong acute tubercles, interstices smooth; end valves with 10-2O 

 such riblets. 



" Interior. — Anterior valve having 8, central 1, posterior valve 

 9 slits; teeth normal; sinus moderate; with about 15 denticles- 

 Girdle furnished with large and small, wide, distinctly striated,, 

 elevated, acutely pointed scales. Length 23J, breadth 12£ mm. r 

 divergence 100°." 



The shell I have selected as the type measures 19 x 9 mm., but 

 the girdle is not well spread out; another given to me by Mr. 

 A. F. Basset Hull measures 19 x 11 mm. It will be seen that 

 these are slightly smaller than the measurements given by Pils- 

 bry, and the number of ribs are proportionately less. Some of 

 the ribs in the end valves bifurcate in my specimens. The rib- 

 bing in the central areas differs slightly from normal tricostalis of 

 a similar size, in that they are narrow ridges, equally raised on 

 both sides, whereas in tricostalis one side is more sloping, more 

 of the nature of " weather boarding," but in both species there is- 

 a considerable range of divergence. 



Variation. — The strong acute tubercles of the lateral ribs are 

 much modified in some specimens. Instead of being " sharp 

 pointed tubercles," they are mere ridges, in this respect approach- 

 ing more closely to tricostalis. Carpenter's manuscript name, 

 carnosus probably referred to this varient. In the specimen I 

 have chosen as type, these tubercles are characteristically sharp 

 pointed. In colour, there is also a considerable variation; one 

 specimen has end valves, the whole of the ridge, and several, 

 lateral areas, pink, two valves have dark brown lateral areas 

 and two cream. Another, not now in my collection, was mostly 

 cream, touched up with pink. 



Habitat. — All my specimens came from Port Jackson ; the one 

 I have selected as type I collected in the Quarantine Station there 

 on 23rd November, 1918. 



The type remains in my collection for the present, but it is 

 intended that it shall ultimately be placed in the South Austra- 

 lian Museum. 



Note. — This shell can easily be distinguished from R. tricostalis r . 

 Pils., and aureo-maculata, Bed. and Mat., by the pointed girdle 

 scales, this feature being so prominent that it can be noted with- 

 out the aid of a lens. 



