356 CEYLON PEARL OYSTER REPORT. 



The mantle is not very large, reaching beyond the body, both laterally and 

 posteriorly, for about 1 millim. and rather more over the gill. The foot is somewhat 

 wider than the mantle laterally, and reaches considerably beyond it posteriorly. The 

 tail is somewhat pointed and bent upwards, terminating in a little conical caudal 

 papilla. The frontal margin of the mantle is beset with two rows of minutely 

 tuberculate papillae ; there are 7 of these in the upper row and 6 in the lower. 

 There are also very small scattered intermediate tubercles along the margin, which 

 is laterally produced on either side into a tentacular appendage. The gill has 

 36 lamellse, each of them pinnate ; it reaches to the level of the end of the mantle. 



The rhinophores as usual rise from the mantle and are split down the outer margin. 



The surface of the mantle is not distinctly tubercular, but is furrowed and 

 rough, owing perhaps to contraction. The foot, mantle and body walls are very 

 muscular and tough. The intercrossing of the muscle bands gives the appearance of 

 a woven material, particularly on the inner surface of the foot after removal of the 

 viscera. 



The genital openings are separate. The vaginal pore has a short tubular opening 

 at the commencement of the gill rachis. The penis is retracted, its opening directed 

 forwards and lying about 1 millim. in front of the female opening. The anus lies 

 dorsal to the membrane which attaches the free end of the branchial plume. 



The jaws are made up of hexagonal rods (Plate V., tig. 27), the edges of the inner 

 marginal rods being very irregularly denticulate. Teeth of radula (Plate V., figs. 26 

 and 28) in about 29 rows, formula 56-0-56. No sign of a median tooth was seen. 

 Form, as figured by Bergh for P. brocki (11). Shell absent. 



The colour of the Ceylon specimen differs considerably from that described by Bergh, 

 which was of a reddish -brown with darker reticulations. Vayssiere mentions a 

 specimen of the same colour, but says that the pigment is superficial, and easily 

 rubbed off, which may account for its absence on this occasion. 



Pleurobranchus (Berthella) citrinus (Rupp and Leuck.). Plate VI, figs. 7 to 10. 



There are 3 specimens of Pleurobranchus apparently belonging to this species 

 which were taken at Jokkenpiddi Paar, Cheval Paar and to the South-east of East 

 Cheval respectively. They differ slightly in colour and appearance, the Jokkenpiddi 

 specimen being of an opaque brownish-yellow and considerably more contracted than 

 the others, which are white, rather translucent and plump. The South-east of East 

 Cheval Paar specimen, which is the best preserved of the 3, measures 1 7 centims. by 

 T05 centims. Its height is 0'5 centim. The mantle is elevated, somewhat inflated, 

 a little wider than the foot, covering the head, and slightly emarginate in front. Its 

 colour is a transparent white with opaque white dots. When examined under a lens 

 of moderate power, the white spots are seen to be subdermal opaque flask-shaped 

 structures tapering towards the surface and surrounded by distinctly marked clear 

 hexagonal areas. The surface of the mantle is smooth. The gill is short and 



