ODHNER : ANATOMY OP SOME BRITISH PISIDIA. 161 



Its pericardial tube has become somewhat more winding, though 

 not in such a degree by far as we find it in P. nitidum (Text-fig. 8). 

 Beside this, the dorsal lobe, which completely covers the pericardial 

 coil of the tube, is more deeply incised by a curved fissure (cf. Text- 

 fig. 1), a feature that we meet with also in P. casertanum. In other 

 species of Eupisidium this partition of the dorsal lobe has given 

 rise to other aspects ; thus in P. amnicum the fissure extends 

 straight backwards (Text-fig. 5) ; in P. nitidum (Text-fig. 8), P. 

 milium, and P. henslowanum it is widened to an open foramen, 

 in which the top of the pericardial tube is visible surrounded by the 

 narrowed annuliform dorsal coil. 



The final stage in this series of the complicated tubuliform 

 nephridium is represented by Sphcerium and Caliculina (cf . Odhner, 

 1912). 



Stomach. — In the alimentary canal we find that the present species 

 is furnished with a more or less distinct posterior coecum, as well as 

 a left one. The intestine describes, in front of the duodenum, a short 

 coil towards the left before it turns and traverses the latter on its 

 right side (Text-fig. 10). 



EXPLANATION OF PLATE III. 



Fig. 1. — P. clessini, Surbeck. Llyn Cwm, Clyd Nant Ffrancon, Snowdon, 



Wales. Coll. Ch. Oldham, 17/9/1921. X 40. 

 Fig. 2. — P. clessini, in adult specimen with fry in gills, showing foot and 



siphon prdtracted. X 40. 

 Fig. 3. — P. torquatum, Stelfox. Grand Junction Canal, Cheddington, 



Buckinghamshire. Coll. Ch. Oldham, 16/7/1921. X 45. 

 Fig. 4. — P. personatum. Malm. Tring, Hertfordshire. Coll. Ch. Oldham, 



16/7/1921. X 40. 

 Fig. 5. — P. clessini, right gill from inside, a, giU axis ; e, interbranchial 



septum; m, marginal vessel. 

 Fig. 6. — P. torquatum, right gill from inside. 6a, Section along a filament 



showing the marginal vessel (m), septa (s), and free edge of reflected 



lamella (l). 

 Fig. 7. — P. personatum, right gills from inside. 7a, Section along a filament 



showing the coalescence of its branches. 



