358 CEYLON PEARL OYSTER REPORT. 



ment of the gill and to an equal distance on the other side. The pedal gland 

 is well developed in the Galle coral-reef specimen, but hardly noticeable in the others. 



The tentacular shield is well developed, but not produced into tentacles laterally. 

 The anterior portion of the lateral margins is grooved. The rhinophores are mode- 

 rately long and split as usual. The gill is moderately long, 1 centim. in the largest 

 specimen, reaching to the end of the body, bipinnate, the rachis with two rows of 

 tubercles, pinnae 25-26, pinnules about 16. 



The genital openings are close together at the base of the gill and appear to be 

 surrounded by a common investment. The shell (Plate VI., fig. 1) is very thin, 

 with an opaque white calcareous coating very fragile and easily rubbed off. It seems 

 to be very faintly striated in the lines of growth. It is situated at about the 

 anterior third of the mantle and is distinctly visible through it as a white patch ; its 

 length in a specimen of T5 centims. is 2 '9 millims. by 1'9 millims. The shell 

 has an ill-defined membranous edge without calcareous coating. The radula in one 

 specimen examined consisted of 46 rows of about 70-0-70 teeth, the inner teeth being 

 simply hooked (Plate VI., fig. 3), the outermost rod-like (Plate VI, fig. 2), slightly 

 bent. The jaws are rather broader than usual, measuring 1'12 millims. by 2 '3 5 millims. ; 

 they are made up of plates of the usual form (Plate VI., fig. 4), with two lateral 

 denticles at the apex, a large distal and a smaller proximal one. 



APPENDIX. 



FAMILY : ONCHIDIID^. 



Onchidium verruculatum, Cuv. Plate VI., figs. 13 to 22. 



There is one specimen of this widely distributed species, very well preserved, from 

 the Gulf of Manaar. The length is 3'1 centims., width 3 '4 centims., height 2 centims., 

 width of foot 1'5 centims. The colour of the preserved specimen is grey, with 

 irregular purplish-brown blotches over the back. 



The back is completely covered with simple and compound tubercles of differing 

 sizes. The simple tubercles are most numerous and vary in size from an extreme 

 minuteness to about 1*2 millims. in diameter. The compound tubercles measure 

 about 1'5 millims. in diameter and are made up of from 5 to 7 simple tubercles. On 

 the posterior fourth of the back are about 10 branchial tubercles, which appear to 

 be made up of from 20 to 40 short papillae crowded together. These apparently 

 represent frondose processes in the living animal. There are only 6 ocular tubercles 

 in the Ceylon specimen, all situated on the anterior half of the body. They bear 

 from 2 to 5 ocelli in a slightly depressed pit on their summit. The general effect of 

 the tuberculation resembles that seen in Archidoris tuberculata. 



The loop of the intestine follows the same course as figured by PLATE for the 

 species, being much less curved than in the closely allied 0. tumidum. 



