404 



Mr. G. C. Rcbson on the 



Jrlypsolia at once from Paludestrina in tliis respect. The 

 autlior is not inclined, liowever, to regard it as of very great 

 taxonomic importance. The points of agreement between 

 the two genera are so numerous that the specialized nature of 

 the respiratory apparatus in Ihjpsobia may, on the whole, be 

 disregarded for classificatory purposes. 



As in Littorina (8) and llemihia (5), the filaments of the 

 gill are prolonged across the roof of the matitle-cavity to the 

 rectal border. This modification of the gill has been very 

 fully discussed by Pelseneer {loc. cit.) ; and according to his 

 description the condition seen in Hypsohia and Hernihia is 

 comparable exactly to the stage illustrated by Littorina, in 

 which each filameut is reduced in height and continued right 

 across the pallial cavity, but has not yet broken up into the 

 vascular arborizations seen in Ceritlddta. 



Fig. 1. 



H. ncso2:hora ( \^ horn. imm. x 4 oc), 

 Section tlirougli two gill-lam el Ice. a, afferent vessel ; e, efferent vessel. 



I propose to refer to the ^' filaments " or " lanielipe " of the 

 true gill, and to call the ridges by which they are continued 

 " paractenidial " folds or filaments. There are from forty- 

 eight to fifty-four lamella?. Each of these in side-view 

 ap]iears as an oblong, slightly bent plate tapering to a 

 blunt poiiit at the right-hand end. This end is actually 

 free, though supported by the paractenidial filaments. The 

 vas^cular system of each filament consists, as usual, of dorsal 

 afferent and ventral eflfeient vessels, with an intermediary 

 system of lacuna; (fig. 1). In general, the condition seen 

 resembles that in Bytlmiella dimktri (3) and P. venirosa (12), 

 though the walls are much thicker and tlie spaces more 



