190 CEYLON PEARL OYSTER REPORT. 



They vary somewhat both in size and arrangement. An account of their minute 

 structure will he found below. 



The Colour is a dull grey with irregular, oval ring-like markings on the dorsal 

 surface and the bases of the arms. 



Localities : Station IT. or III., west of Ceylon. Three specimens [213-215]. 



Cheval Pearl Banks, Gulf of Manaar. Six specimens [212, 239-243]. 



Station XVI., on Periya Paar ; depth 9 fathoms. One specimen [217]. 



I am not accpiainted with any other species at all resembling this one in its surface 

 decoration. 



The fact of the proximal suckers being in a single row, not only in arms which are 

 bent outwards, but also in those which are straight, is of some interest. The 

 arrangement of these suckers has often been made use of as a specific character, but 

 has always appeared to me to be of very doubtful validity, because there is no doubt 

 that in many instances it varies with the curvature of the arm. The present instance 

 suggests that it may have more value than 1 have hitherto supposed. 



Structure of the Branched Papillae. 



The Papillae A'ary a good deal in dimensions. An average one (Plate II., fig. 8) 

 would be 0"75 millim. high by 0'4 millim. in diameter, whilst a large one (Plate II., 

 fig. 9) would attain 1*65 millims. in height by O'G millim. in diameter. Each consists 

 of a stem and branches, both of which are plentifully bestrewn with chromatophores. 

 Between the larger papilla? are small ones of various degrees of complexity ; some are 

 simple, some bifurcated, some with three or more branches. Papillae of the smaller 

 kinds are found on the bases of the arms. 



The Stem is conical in form, arising gradually from a broad base. In the smaller 

 papillae (Plate II., fig. 8) the stem remains unbranched to the apex and there gives 

 rise to branches, from two or three to about half a dozen in number. In the larger 

 (Plate II., fig. 9) the branches are given off at intervals from near the base. 

 Sometimes the apex is seen to persist independently of the branches ; in other 

 instances there is no definite apex, but the stem divides into a tuft of branches at the 

 top. The apex sometimes presents a yellowish aj)pearance as though a yellow internal 

 mass were shining through the integument. 



The Branches are almost cylindrical, tapering only very slightly towards the tips, 

 which are bluntly rounded off. Their diameter varies from 0'03 millim. at the tip to 

 0'15 millim. at their junction with the stem, and their length from 0"8 millim. to 

 0*25 millim. In most cases they are simple, but in a few instances they bifurcate 

 (Plate II., fig. 8), and still more rarely trifurcate (Plate II., fig. 9). 



When a series of sections is examined, it is found that in those taken near the base, 

 just above the muscles of the body wall, there is in the centre a mass of tissue 

 (Plate III., fig. 1, cm,) from 0'21 millim. to 0'24 millim. in diameter. It stains but 



