KR. BONNEVIE 



[REP. OF THE "MICHAEL SARS" NORTH 



Systematic part. 



EUTHECOSOMATA Meisenheimer. 

 Limacinidae. 



The various species of this family are known (Pelse- 

 neer 1888, 1906, Meisenheimer 1905) to belong to three 

 different genera, viz. : Limacina, Peraclis, and Procymbulia, 

 all characterised by a spirally twisted body, but otherwise 

 differing from each other on essential points. 



the base of development of the "pseudothecosomatous" 

 family Cymbuliidae. 



Pelseneer (1906) on the other hand maintains that 

 the genus Peraclis must be considered the most archaic 

 group among living thecosomatous pteropods, standing 

 (pag. 148) "tout a la base de l'arbre phylogenetique des 

 'Pteropodes thecosomes'". This conclusion is drawn from 

 the fact that in Peraclis triacantha Pelseneer has found 



Mil yv 



H 



. M 



Fig. 1. Mantle-margin with ctenidium and balancer of Peraclis triacantha A., P- diversa B. and P. reticulata C. 



In O the ctenidium and the osphradium (?) are drawn at a higher power, b. balancer, ct, ctenidium. 



p- g. paliial gland, o. (os) osphradium (?). gl. I. glandular lobe. 



The natural relations between these genera have been 

 discussed by the two authors mentioned, and this question 

 being of importance for our view on the relationship 

 within the whole group of thecosomatous pteropods, it 

 will be discussed also in this paper. 



According to Meisenheimer the genus Limacina seems 

 to form the base of one line of development within this 

 group leading over to the family Cavoliniidae, while 

 Peraclis together with the new genus Procymbulia form 

 the paliial cavity situated not quite dorsally but on 



the right side of the body and has also proved the 

 existence of a normal ctenidium like that occurring 

 in archaic forms of the tectibranch group Bulloidea, the 

 group of opisthobranch molluscs to which the thecoso- 

 matous pteropods seem to be most nearly related. 



As will be shown below, this view of Pelseneer is 

 strongly supported by my results from other species of 

 Peraclis, and consequently this genus will be described 

 first as the most primitive of the Thecosomata. 



