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



Key to the Species. 



I. Radiating crests of the posterior gill long, with subdivided and 

 numerous fringes. 



1. Each acetabuliferous appendage bearing seven large suckers 



inserted on the margins of the appendage, . . Pneurnonoderma mediterraneum. 



2. Each acetabuliferous appendage bearing numerous small 



suckers on their whole median face. 



A. Each appendage with about forty suckers, . . Pneurnonoderma violaceum. 



B. Each appendage with about one hundred suckers, . Pneurnonoderma peroni. 

 II. Radiating crests of the posterior gill short, with simple and not 



numerous fringes. 



1. Fringes well marked, strongly separated, and shrunk at their 



tase, ....... Pneurnonoderma boasi. 



2. Fringes slightly developed and forming merely simple, 



continuous and slightly projecting undulations. 



A. Triangular, rather short acetabuliferous appendages, 



each bearing about fifty suckers, . . . Pneurnonoderma pacificum. 



B. Very long acetabuliferous appendages with nearly 



parallel margins, each bearing about seventy-five 



suckers, ...... Pneurnonoderma souleyeti. 



1. Pneurnonoderma mediterraneum, van Beneden (PI. I. fig. 8 ; PI. II. fig. 1). 



1838. Pneumoderrnon mediterraneum, van Beneden, Note sur une nouvelle espece de Pneumo- 



derme, Mem. Acad. Sci. Bruxelles, t. xi. p. 13, pi. 

 hi. figs. 1-2. 

 1852. Pneumoderrnon peronii, Souleyet (pars), Voyage de la Bonite, Zoologie, t. ii. p. 274, 



pi. xiv. figs. 13, 1 4, 16. 

 1855. Pneumoderrnon mediterraneum, Gegenbaur, Untersuchungen fiber Pteropoden und Hetero- 



poden, p. 213. 

 1859. Spongiobranchea australis, Chenu (pars), Manuel de Conchyliologie, t. i. p. 116, fig. 509. 

 1873. Pneumoderrnon peronii, Costa, Pteropodi della Fauna di Napoli, p. 22, pi. v. figs. 1-3. 

 1886. Pneumoderrnon macroeotylum, Boas, Spolia atlantica, K. dansk. Vidensk. Selsk., Skriv., 6 



Raekke, Bd. iv. p. 152. 

 1886. Pneumoderrnon audebardi, Locard, Prodrome de Malacologie franchise, Mollusques 



marins, p. 19. 



Body rather long. 



Foot- — Posterior lobe very long (no other known Pneurnonoderma has it so long). 



Lateral Gill rather short, with fringes on the lateral somatic crests. 



Posterior Gill. — The four radiating crests are long, and possess on each side of their 

 base about seven ramified fringes ; such fringes are also to be found on the anterior and 

 posterior sides of the quadrangular crest. 



Acetabidiferous A-ppcndages rather long and not very wide at their base ; each bearing 

 generally seven (sometimes six) suckers inserted on the median side of the appendage, 

 the four largest on the abdominal margin, and the three smallest at the distal extremity 

 and on the dorsal margin ; the latter are sometimes very small. The shape of the 



