340 CEYLON PEARL OYSTER REPORT. 



opening is circular and measures about 0'6 millim. in diameter, its edges are thin 

 and raised into a collar with smooth even margin. The rhinophoral pores are oval, 

 placed longitudinally with even edges somewhat raised. The pseudo-peritoneum is 

 of a blackish-brown colour. The foot is moderately wide : in the preserved specimen 

 it is much contracted and measures 0'45 centim. across. It is bi-lipped in front, the 

 anterior lip bifid. The head is small, the tentacles very small and apparently 

 tubercular. 



The branchiae (Plate III., fig. 6) are 4 in number and tri-pinnate, the anterior 

 pair larger, the posterior pair shorter and divided deeply into two main branches. 

 The rachides are rather thick, white on the outer side and black interiorly. The 

 lamellae are black. The anus is rather long and tubular, white with a black crenulate 

 margin. The rhinophores are stout, with black lamellated tips and thick white base. 



The radula sac is unusually long and curved towards the left side. The radula 

 consists of 50 rows of hooked teeth ; the lateral teeth on each side number about 50, 

 a median tooth being absent. The 20 innermost teeth (Plate III., fig. 5) are very 

 small, increasing outwards ; the 3 outermost teeth (Plate III., fig. 7) are small and 

 thin, but not fringed or denticulate. There is no buccal armature, but the buccal 

 tube has a stiff chitinous lining in which were embedded several grapnel-shaped 

 sponge spicules. 



The penis and vagina are as far as could be made out unarmed. The stomach and 

 gut were both crowded with tapering rod-shaped spicules roughened at the apex and 

 slightly bent at the base. 



This species agrees with BEEGH'S diagnosis of the genus Halgerda in the following 

 particulars : The smooth leathery mantle raised into tubercular ridges, the bi-lipped 

 foot, small tentacles, small number of branchiae, absence of jaws, long curved radula 

 sac, radula without median tooth and with the innermost teeth markedly smaller, 

 unarmed penis and vagina. The genus Dictyodoris, though placed by BERGH in a 

 different- sub-family , also approaches it in some respects. 



Halgerda formosa from Mauritius differs from the species now described in having 

 more numerous and apparently narrow dorsal ridges, and in having two outermost teeth 

 of the radula on either side denticulate. It is marked with crimson spots and yellow 

 lines, and Professor HERDMAN states that the black spots found on H. punctata are 

 purple during life. 



Thordisa (?) caudata, n. sp. Plate II., figs. 18 and 19. 



I have provisionally placed this species in the genus Thordisa, though ohe of the 

 most noticeable features of the genus, the villose mantle, is absent. 



A single specimen was taken to the south-east of Cheval Paar in February, 1902. 

 The length of the animal is 2'9 centims., width I'l centims., height 1'6 centims. ; 

 the contracted foot is 0'8 centim. wide. The colour is a creamy white without 

 markings, the rhinophores grey, the branchiae white. 



