144 THE VOYAGE OF H.M.S. CHALLENGER. 



anterior is also wide, and is flattened or slightly concave in the middle between the aper- 

 tures. The dorsal and ventral edges are short and strongly convex, and the body is 

 attached slightly by the left side and the posterior end. The apertures are both on the 

 wide anterior extremity; they are moderately distant, slightly projecting, large, and 

 four-lobed. 



The surface is even but slightly roughened, and has a few shell fragments, &c, 

 adhering here and there. The colour is a warm yellowish-brown. 



Length of the body (antero-posterior), 1*5 cm. ; breadth of the body (dorso-ventral), 

 2-3 cm. 



The Test is cartilaginous, but thin and soft ; it is semi-transparent, and is well 

 supplied with blood-vessels with enlarged terminal knobs, and containing numerous short 

 rod-like echinated spicules in their walls. 



Tlie Mantle is thin, and of a light brown colour. The musculature is feeble but 

 distinct. It contains very delicate elongated fusiform spicules. 



The Branchial Sac has seven folds on each side. There are four internal longitudinal 

 bars on the fold, and two in the space between the folds. The transverse vessels are all 

 of much the same size, and delicate fusiform calcareous spicules are present in abundance 

 in their walls. The meshes are square or elongated transversely, and are divided by a 

 delicate membrane. They each contain about six stigmata. 



The Dorsal Lamina is represented by a series of rather small and closely placed 

 tentacular languets. 



The Tentacles are not large. There are fifteen larger and fifteen smaller compound 

 ones placed alternately, and in addition there are about thirty minute simple processes, 

 which alternate with the branched tentacles. 



The Dorsal Tubercle is small and simple, ovate in form, and placed at the bottom of a 

 triangular peritubercular area ; the aperture is anterior and minute ; the horns nearly 

 meet, but are not coiled. 



This little species is closely allied to Cynthia pallida, Heller, with which I classed it 

 provisionally in the Third Preliminary Report (see Proc. Eoy. Soc. Edin., 1880-81, 

 p. Gl); the two species differ, however, in several particulars. According to Heller, 1 

 Cynthia pallida is also found at Tahiti. 



The two specimens from 20 fathoms differ slightly in external appearance (PI. 

 XVII. figs. 10 and 11) from the other five. They are not elongated transversely, and 

 the atrial aperture is more dorsal in position (fig. 10). In colour also they differ, 

 being milk-white in place of yellowish-brown. At first I was inclined to consider them as 

 a distinct species, but as a detailed examination has revealed no structural differences, 

 it is probable that they are merely individual variations, or at most a local variety. 



1 Beitruge zur niihern Kenntniss Jer Tunicaten, Sitzb. dor k. Akad. der Wksensch., Bd. lxxvii., 1878, p. 15. 



