Massy — The Gymnosomatous Pteropoda of the Coasts of Ireland. 225 



group, the key given below is not based on any particular system, but is 

 merely intended as a means by which the species mentioned can be readily 

 separated from one another. All the genera yet known are enumerated. The 

 sizes given do not define the known limit of growth, but indicate that the 

 characters noted in the key have been observed in specimens of that age. 



The following genera and species included in the key have not been 

 recorded from British and Irish waters, but it seems probable that some of 

 them may yet be found : — 



Pneumoderma. 



Schizobrachium. 



Paraclione. 



Thalassopterus zancleus, Kwiet. 



Halopsyche. 



Fowlerina Hjortii, Bonn. 



Microdonta longicollis (Bonn). 



Key for the Identification of British and Irish Gymnosomatous Pteropods. 



Suckers present (1). 

 Suckers absent (2). 



(1) Median sucker group present (3). 

 Median sucker group absent (4). 



(3) Median group with 4 lateral and 1 terminal suckers on an append- 



age (5). 

 Median group with 2 suckers not on an appendage (6). 

 Median group with 5 small suckers on long stalks not on an 



appendage = Pneumodermopsis macrochira, Meis., 6-8 mm. 



(5) Distal paired suckers of median arm greatly enlarged, and each 



armed with a spine = Pneumodermopsis ciliata (Ggbr.), 

 2-50-15 mm. 

 Distal paired suckers of median arm not enlarged and without 

 spines = Pneumodermopsis paucidens (Boas), 2-4-50 mm. 



(6) Lateral sucker-bearing arms present. Lateral teeth of radula rela- 



tively short = Pneumodermopsis Michaelsarsi, Bonn., 3-7 mm. 

 Lateral sucker-bearing arms absent. Lateral teeth of radula rela- 

 tively long = Pneumodermopsis oligocotyla, sp. n., 4-5 mm. 



(4) Median tooth of radula absent in adult = Pneumoderma. 



Median tooth of radula present in adult with 3 denticles (7). 



Median tooth of radula present in adult with 2 denticles = ScJiizo- 



brachium. 



2o2 



