72 



measures, length 8 mm., breadth 2 mm. I have specimens that 

 measure 12 mm. by 2*75 mm., girdle recurved. 



Locality. — I have found this species on the cylindrical 

 stems of Cymodocea antarctica at Marino, Normanville, Cape 

 Jervis, and Encounter Bay, all jn South Australia, and Dr. 

 Torr has specimens found in shell-sand at Aldinga. 



Specific name. — I have designated this species under the 

 specific name of cymodocealis, after the generic name of the 

 plant that is its host, and probably its food plant also. The 

 plant was named after a sea nymph. 



Variation. — While the type is green all over, the decora- 

 tion being due to either lighter or darker shades of the same 

 green, the species is subject to considerable variation both in 

 colour and markings. In one from Normanville, the apex of 

 the anterior valve and the blunt beak of the next four valves is 

 bright pink, also the ground-colour of pale olive that is present 

 in most is in this specimen replaced with silvery-green, the 

 markings mottled instead of striped, and the posterior margins 

 of each valve decorated with 1 to 3 blackish dots, which without 

 the aid of a lens look like pits. One other specimen from this 

 locality has a dark-pink line the whole length of the dorsal 

 area. 



Remarks. — This shell is easily distinguished from any 

 other known Stenochiton by its highly arched character. The 

 general appearance is that of a canoe turned bottom upwards. 

 This effect is added to by the incurved girdle. In life, with 

 the exception of the anterior and posterior portions, the whole 

 of the girdle curves round clasping the hard cylindrical stem 

 of the host plant, sometimes meeting on the other side. While 

 the animal is able to flatten out the girdle enough to creep 

 about on the surface of a bottle, it is evidently an abnormal 

 position. In endeavouring to flatten out the girdles for 

 figuring purposes I spoilt several specimens, and have, I fear, 

 expanded the valves somewhat, giving a total width beyond 

 what is true to nature. 



I am indebted to Mr. F. L. Saunders for pointing out to 

 me the host plant. He had previously found specimens at 

 Normanville, and wrongly identified the shell as Stenochiton 

 pallens, Ashby. 



I am presenting the type and other specimens figured to 

 the South Australian Museum. 



Stenochiton posidonialis, n. sp. 

 Pis. xiii. and xiv., figs. 2, 6, and 13 (a, ?), c, d ). 

 General appearance. — Shell long, flat, rounded, smooth, 

 . and highly polished. The whole shell is exceptionally flat as 

 compared with other members of this genus. 



