REPORT ON THE PTEROPODA. 25 



Up to the time of d'Orbigny (1840), the Pneumonoderma of tlie North Atlantic 

 (Pneumonoderma violaceum), was described under the same name ; and under this name 

 also, Souleyet * reunites some different Pneumonoderma, of which not one was the true 

 Pneumonoderma peroni. 



It is true that in 1815 Oken 2 had distinguished the two forms (Pneumono- 

 derma peroni and Pneumonoderma violaceum), and had respectively named them JEgle 

 cucullata and Pneumodermis atlantica; but the diagnoses he gave of them are 

 so brief and incorrect that these names cannot be retained, and must be regarded as 

 synonyms. 



The two other names to retain with Pneumonoderma peroni and Pneumonoderma 

 violaceum, are Pneumonoderma mediterraneum, van Beneden, and Pneumonoderma 

 pacificum, Dall. 



Thus, among the ten different specific names above mentioned, there are only four 

 which can be considered as definitive in the actual nomenclature. Besides these four 

 species I shall describe two new ones, Pneumonoderma souleyeti and Pneumonoderma 

 boasi, of which the former was collected by the Challenger Expedition. 



As I have already said, the different species of Pneumonoderma are very difficult to 

 distinguish from one another. The distinctive character proposed by Boas is founded 

 on the number and form of the suckers of the buccal appendages ; but this character 

 is not sufficient for all the species. By adding to it the character drawn from the 

 conformation of the gills, one can much more easily distinguish the six species above 

 mentioned. 



By the conformation of the posterior gill, these six forms are divided into two groups, 

 each including three species : — 



I. In the first group the radiating crests of the posterior gill are one-fifth of the 

 length of the body, and bear numerous fringes (from seven to twelve), which are long and 

 subdivided. The species of this group generally have the free part of the lateral gill 

 rather short ; one of them does not possess fringes on the lateral somatic crest of this 

 gill. 



II. In the second group, the radiating crests of the posterior gill are short (one-tenth 

 of the length of the body), and bear a few fringes (about three), little projecting, and 

 simple. Two species of this group (Pneumonoderma pacificum and Pneumonoderma 

 souleyeti) have the free part of the lateral gill long, and the fringes of the posterior gill 

 little developed. 



The following table, combining the characters taken from the gills with those of the 

 acetabuliferous appendages, allows of the easy discrimination of the six species of Pneu- 

 monoderma described in this Beport. 



1 Voyage de la Bonite, Zoologie, pi. xiv. figs. 7-16. 



2 Lehrbuch der Zoologie, Bd. i. p. 327. 



(ZOOL. CHALL. EXP.— PART lviii.— 1887.) Mmm 4 



