ODHNER : ANATOMY OF SOME BRITISH PISIDIA. 159 



gill, are connected to the opposite gill by means of an interbrancMal 

 septum exactly as in P. dessini. 



After its flexure the ascending branch of the filaments is fused to 

 the descending one for nearly the whole of its length ; only the 

 marginal end projects freely (PI. Ill, Fig. 6a). 



The number of filaments in the gill is about twenty-five, and the 

 interfilamentar junctions are about eight. 



Mantle. — As to the mantle, the pedal slit is very long and the 

 post-pedal suture correspondingly short, about half the height of 

 the posterior adductor or the length of the anal slit. I have not 

 observed any rudimentary branchial slit like that occasionally 

 occurring in P. dessini. 



Nephridium. — The nephridium (Text-fig. 2) has essentially the 

 same shape as in P. dessini ; only it is lodged somewhat lower on 

 the front side of the adductor. 



Stomach. — The stomach has no posterior pocket but a well- 

 developed left coecum, and the intestine is rather short and describes 

 a curve to the left, as in P. personatum, before traversing the 

 duodenum on its right side. 



Though agreeing in essential characters, the two species just 

 mentioned differ, however, in several respects. To the anatomical 

 differences, especially in the structure of the gill, conchological ones 

 are added : the very feebly constructed shell of P. dessini contrasts 

 greatly with the solid valves of P. torquatum, which are, further, 

 equipped with the umbonal crest, a rather enigmatic phenomenon. 

 In consideration of these facts it seems questionable whether the 

 two species have a common origin, and it should be emphasized 

 that the creation of a subgenus Neopisidium for comprising them 

 implies only their systematic but not genetic unity. Their agreeing 

 in simpler organization has been attained, probably, by reduction, 

 and this process might have proceeded diphyletically. Further 

 inquiries only can bring decision. 



PiSIDIUM PERSONATUM, Malm. 



Gill.—l have chosen this species as a representative of the 

 subgenus Eupisidium because it differs less than other species from 

 the types described above, and thus, in some respects, serves to 

 bridge the transition. We find here the typical two gills on each 

 side of the body, the larger or anterior one corresponding to the single 

 one in P. dessini and torquatum, as well as to the anterior (or inner) 

 one in Sphcerium and other Eulamellibranchia. This gill (PL III, 

 Fig. 7) consists of a well-developed direct lamella and a smaller 

 reflected one, which ascends to about half the breadth of the former. 

 The margin of the reflected lamella is attached along the side of the 

 body for more than two-thirds of its length ; then, behind the foot, 

 the margins of opposite lamellae are united to form an interbranchial 

 septum. 



