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



Radula. — Formula 3:1:3; the median tooth is tricuspid. 



Hook-sacs. — Each with about ten rather long hooks. 



Colour, brownish. 



Length, 5 mm. The two posterior ciliated rings remain until a late period. The 

 three rings were still visible in a specimen 3 mm. long. 



Habitat. — The type-specimens, described by Boas, were gathered by the Italian ship 

 " Vettor Pisani," in the South-East Pacific, off Caldera (Chili), about lat. 27° S., February 

 18, 1882. 



In a part of the collection of this scientific expedition, which had not been seen by 

 Boas, I have found larvae of a species of Gymnosomatous Pteropod, caught off Panama 

 about lat. 8° N, December 1883 (PL I. fig. 5), among which the oldest shows the peculiar 

 acetabuliferous appendages of Dexiobranchsea polycotyla. Thus the geographical 

 distribution of this species embraces the whole South-West Pacific, along almost the 

 whole west coast of South America. 



Challenger Specimens. — The Challenger specimens which belong to this species, are 

 the very young larvae of Gymnosomata described by H. N. Moseley, 1 from Station 297, 

 November 11, 1875, Tahiti to Valparaiso, lat. 37° 29' S., long. 83° 7' W. These larvae 

 differ from those of Pneumonoderma, and resemble much that of Dexiobranchsea ciliata, 

 described by Gegenbaur. 2 Besides, the abundance of Dexiobranchtea in this region en- 

 ables me to place these larvae with great certainty in this last genus. 



As these specimens have not been preserved, and since the formula of their radula 

 has not been recorded, it may seem difficult to determine specifically larvae that are so 

 young. Nevertheless, it is nearly certain that they belong to the present species, not 

 only on account of its extensive geographical distribution, of its comparative abundance, 

 and perhaps of the elongated form of the larva, but chiefly on account of the length of 

 the posterior lobe of the foot. 



Spo7igiobranchsea, z d'Orbigny. 



1825. " Cliodita," Quoy and Gaimard (pars), Description de cinq genres de Mollusques, Ann. d. 



Sci. Nat., s£r. 1, t. vi. p. 74. 



1840. Spongiubranchea, d'Orbigny, Voyage dans TAme'rique meVidionale, t. v. p. 130. 



1842. Spongiobranchia, d'Orbigny, Pal^ontologie frangaise, terrains cretact5s, t. ii. p. 4. 



1856. Spongiobranchxa, Woodward, A Manual of the Mollusca, p. 209. 



1862. Spongobranchia, Bronn, Die Klassen und Ordnungen des Thierreichs, Bd. iii. p. 645. 



1885. Spongobranchus, Wagner, Die Wirbellosen des weissen Meeres, Bd. i. p. 120. 



Body ovate, rather long, round posteriorly ; visceral mass extending to the posterior 

 extremity of the body ; skin rjigmented. 



1 On Styloclms pelagieus, &c, Quart. Joum. Micr. Sci., 1877, p. 32, pi. iii. fig. 14. 

 - Untersuclnmgen iiber Pteropoden und Heteropoden, pi. iv. fig. 11. 

 3 From tmo'/yiU, sponge, and /^ay^/a, gill. 



