REPORT ON THE PTEROPODA. 



19 



Foot. — Posterior lobe lengthened, tapering greatly, and pointed at its posterior end. 



Head rather long ; labial tentacles long. 



Gills. — (l) Lateral gill little developed and very short, forming a simple prominence 

 on the right side of the body ; (2) posterior or caudal gill 

 as a simple membranous ring, without exterior radii and 

 fringes or folds at its base. 



Acetabuliferous Appendages. — Two symmetrical appen- 

 dages inserted on the ventral wall of the front portion of the 

 buccal cavity, or protrusible part of the digestive tract ; each 

 appendage bears, on the side which is median during the 

 extension, suckers arranged in a single row, and of which the 

 size increases from the base of the appendage to its distal 

 extremity. 



The protrusible front portion of the digestive tract con- 

 sists of a rather long proboscis (fig. 2, 1), on the ventral 

 side of which, as in Pneumonoderma, is a papilla (fig. 2, 3) anterior (during the 

 evagination of the proboscis) to the acetabuliferous appendages. 



Hook-sacs rather short, like those of Clione, and also containing teeth of different 

 length, slightly bent. 



Radula always with a median tooth in the adult. 



Jaw rather small, with slender spines. 



Since d'Orbigny (1840), whose species australis is alone a true Spongiobranchsea, 1 

 this genus has never been studied till lately by Boas and by me. Souleyet, 2 Gegenbaur," 

 Fischer, 4 &c, have considered it as a Pneumonoderma badly described, and Krohn 5 as an 

 immature Pneumonoderma. It is, however, a very good, distinct, and well-characterised 

 genus, as may be seen from the foregoing description and from the figures. 



Fig. 2. — The proboscis of Spongin- 

 branchcea, expanded ; ventral aspect. 

 Magnified about 8 diameters. 1, 

 proboscis ; 2, hook-sacs ; 3, ventral 

 papilla ; 4, acetabuliferous appen- 

 dages ; 5, labial tentacles ; 6, head. 



* Spongiobranchsea australis, d'Orbigny (PI. I. figs. 6, 7). 



1825. " Cliodita caduceus," Quoy and Gaimard, Description de cinq genres de Mollusques, 



Ann. d. Sci. Nat., ser. 1, t. vi. p. 74, pi. ii. fig. 2. 

 1840. Spongiobranchea australis, d'Orbigny, Voyage dans l'Amerique meridionale, t. v. p. 131, 



pi. xix. figs. 1-7. 



Acetabuliferous Appendages bearing on their median side from eight to ten suckers, 

 with short peduncles. The three or four more proximal are very small, the largest 

 (distal) are inserted on the ventral margin of the appendages, as in Pneumonoderma 

 mediterraneum. 



Radula. — Formula 8:1:8. 



1 Spongiobranchsea elongata, cl'Orb. = Clione longicaudata, Soul. 

 3 Untersuchitngen iiber Pteropoden und Htteropoden, p. 173. 



2 Voyage de la Bonite, Zoologie, t. ii. p. 272. 

 4 Manuel de Concbyliologie, p. 424. 

 Beitrage zur Entwickelungsgeschichte der Pteropoden und Heteropoden, p. 17. 



