298 CEYLON PEARL OYSTER REPORT. 



scutes are the principal features which differentiate this species from Leprea ehren- 

 berr/i (Grube), Marenzeller (' Sudjapan. Ann.,' part ii., 1884, p. 5 of reprint). 



Polymnia labiata, n. sp. Plate VI., figs. 143 to 145. 



Locality : One specimen from the pearl banks, Gulf of Manaar ; another from 

 Aripu Reef. The former inhabited a membranous tube encrusted with coarse 

 unequal sand-grains and calcareous fragments (including an entire small Naticoid 

 shell) ; the latter has a large test-like tube encrusted with foreign particles, mostly 

 very small and chiefly calcareous. It appears that the incrustation of the tube of 

 Terebellids affords no trustworthy evidence of specific identity except in particular 

 instances. The tube in question is nearly six inches long, with a diameter of half an 

 inch. The abdominal region of the contained worm had undergone fragmentation, 

 but the other specimen from the pearl banks is complete ; length about 115 millims., 

 width of thorax 7 millims., segments upwards of 120. The transition from the 

 torigerous to the pinnigerous region is not very abrupt, the diameter of the body 

 gradually decreases and the anterior pairs of pinnae may be rather wide and set low 

 in the integument, resembling the tori. The well-marked scuta ventralia end between 

 the ninth and tenth pairs of tori, leaving six clear pairs of tori behind the scutigerous 

 region. When the ventral wall of the thorax is much contracted there is an 

 appearance of ill-defined scuta being continued to the posterior end of the thoracic 

 (torigerous) region. The tori of the right and left sides are always widely separated, 

 not approximating in the mid- ventral line. 



The diagnostic characters by which this species is to be distinguished from its 

 congeners (in the absence of information concerning colour) are afforded by the 

 structure and proportions of the uncini. The uncinus consists of three principal parts, 

 the hook, the shaft or neck, and the manubrium ; the relative dimensions of these 

 parts appear to be constant for the species. The uncini of P. labiata are noticeable 

 on account of the length and slenderness of the neck ; the basal angle of the 

 manubrium, which lies deepest in the integument, is produced into a short guber- 

 naculum resembling the " Muskelfortsatz " of the uncinus of Pista (v. Marenzeller, 

 ' Adriat. Ann.'). The denticulations of the uncinus offer no tangible distinction and 

 present considerable variety both in number and dimensions. Generally the main 

 hook is surmounted by a pair of smaller hooks of the second order, followed by 

 another row of three denticulations of the third order (the middle tooth usually much 

 longer than the lateral) occupying the vertex of the uncinus. Occasionally accessory 

 denticles may be observed on the brow of the uncinus (Plate VI., figs. 144 and 145). 



No Polymnia was described by Grube from the Philippines. One species, 

 P. congruens, v. Marenz., is known from South Japan, differing clearly from 

 P. labiata in the shape of the uncini, and in the number of scuta ventralia. Grube's 

 Terebella sarsii from the Philippines ('Ann. Semp.,' p. 223) has three pairs of 

 arborescent gills but no lateral lobes, conforming to Nicolea in the latter respect. 



