74 



grooves that coalesce in places, but by holding the shell side- 

 ways a good lens reveals the fact that the shell is absolutely 

 unsculptured. Dorsal area is not beaked. 



Girdle. — Less than '5 mm. in width. A dark blotch 

 extends across the girdle at each suture and irregularly in 

 front of the anterior valve. Is covered with small, closely- 

 imbricating scales, only a portion of their rounded ends being 

 visible, but which are when exposed found to be flattened, 

 elliptical, rounded at ends, straight-sided, about twice as long 

 as wide. The outer two or three rows are drawn out into 

 coarse, transparent hairs or spicules, forming a distinct fringe. 



Measurements. — Type (flat view in figure in plate) : length, 

 9*5 mm. ; width, including girdle, 3'5 mm. 



Localities. — I have found it on the eastern side of Gulf 

 St. Vincent wherever I have searched for it on Posidonia 

 australis; also at Port Lincoln; and Dr. Torr has found the 

 large form exceedingly plentiful at Corny Point on Spencer 

 Gulf. 



Specific name. — I have designated this species under the 

 specific name of posidonialis , after the generic name of the 

 plant which is its host, and probably its food plant. It is 

 always, as far as my experience goes, found near the whitish 

 base of the ribbon-like green leaves of Posidonia australis, just 

 above where the leaves enter the sand. The plant was named 

 after Poseidon, a god of the sea. 



Variation. — While the normal colouration is transparent 

 green to olive-green, flecked or streaked with dark- green 

 markings, in some specimens obtained by myself at Marino, 

 and others collected by Dr. Torr at Corny Point, up to 15 mm. 

 in length, the ground-colour is almost white, ornamented with 

 a V-shaped, dark-brown blotch in anterior and posterior 

 valves, and a V-shaped brown marking covering each dorsal 

 area ; the whole of the 4th valve and lateral area of the 3rd 

 valve also dark brown. In a specimen, 17 mm. long, from 

 Largs, sent me by Mr. E. H. Matthews, the ground-colour is 

 dingy buff, with a V-shaped brown blotch on 1st and last 

 valves, and a brown streak continuing through all the dorsal 

 areas. Dr. Torr has also a similar specimen. In another of 

 Dr. Torr's, the shell is orange colour. 



Remarks. — This shell is easily distinguished from any 

 other known Stenochiton by the shape of the anterior valve, 

 which is distinctly concave, the general flat character of the 

 shell, and the exceptionally flat posterior valve, the mucro being 

 hardly perceptible. The figure in plate showing side view will 

 sufficiently demonstrate these differences. 



I am presenting the type to the South Australian Museum. 



