Fauna Associated with Crinoids of Tropical Coral Reef, etc. 81 
little chela is noticed. It is this feature which distinguishes the type 
species from all the others included in the comatularum group. The 
thumb is much longer than its unmovable fellow and forms a strong 
recurved hook, by means of which, Haswell remarked, it clung to the 
arm of the crinoid. The individuals taken on the Dogai were generally 
seated in pairs on the disc, like S. brucei, but when slightly disturbed 
they immediately dug the hook of this chela into the flesh of the disc, 
from which it was only dislodged with difficulty; or if the alpheid had 
left the surface of the disc, the chela was serviceable for clasping the 
arm of itshost. But this instrument is only used for temporary attach- 
ment, nor is it usually embedded in the host when the animal is at rest. 
The wonderful similarity in colouring between the two species of 
Synalpheus is noticed above. There is, however, one marked difference; 
that is, the entrace of a white or yellow pigment into the colour scheme 
of S. comatularum. <A typical example of a light-coloured crinoid and 
its commensal may be briefly described here. 
The crinoid (C. annulatum) had a yellow-green disc, the arms were black 
with white tips, sometimes with a dash of rusty-red pigment, and the pinnules 
white, sometimes grey at the side. The underside of the arms was yellow- 
green and the cirri were white. Both the commensals were light coloured and 
harmonized to a considerable extent with the host. The female had five 
prominent lines of white pigment on the thorax alternating with brown lines; 
in the abdomen there was a median white line with a brown line on each 
side, most of the median space being colourless and a non-pigmented eye 
being found on each pleuron. The male showed 6 lines of brownish-purple 
pigment on the thorax, all else being colourless, save for some dashes of white 
pigment. In this case the two partners differed slightly; the female showed 
well-developed white pigment and the other pigment was brown; in the male 
white pigment was almost absent and the pattern was formed by a purple- 
brown darker than in the female. 
PONTONIID. 
Three prawns of this family, all new to science, were found to be 
characteristic members of the crinoid fauna. These are: Pontoniopsis 
comanthi, Periclimenes pottst and P. commensalis. They have all been 
examined by Mr. L. A. Borradaile and full diagnoses have appeared in 
his recent revision of the family. I will confine myself here to giving 
the details of their colouration and habits which were noted. 
Pontoniopsis comanthi Borradaile. 
This was found only at Mabuiag on the light-coloured varieties of 
Comanthus annulatum in which a great deal of green pigment had 
developed. In general this small crustacean (it is about 8 mm. long) 
exhibits a striking correspondence with its host. Alternate longitudinal 
stripes of bright green and black or dark brown occur on the dorsum, 
and these match with the crinoid cirri, in which green and yellow alter- 
