114 



Records of the Inclinn Museum. 



[Vol. XV, 



The common canal, formed by the union of tlie two suprabranchial 

 canals behind the visceral mass, is shorter than the antero-posterior 

 len,o;th of the cloacal chamber in Lamellidens and Solenaia, but nearly 

 of the same length in Physunio. 



Structuke of the gills. — The specimens of Solenaia soleniformis, 

 Benson, and Lamellidens marginalis, Lam., supplied me with gravid 

 females, while the specimens of the two species of Physunio were all 

 sterile. In Solenaia all the four gills act as marsupia, while in Lamelli- 

 dens the outer gills only give rise to ovisacs. In Physunio the structure 

 of the gills seems to show that the outer gills are concerned in carrying 

 the embryos. The arrangement of the gill filaments and the formation 

 of lateral tubes are shown in the followinti table : — 



Number of gill-filaments corresponding to 

 one water tube or lateral tube. 



Solenaia — 



Outer gill, 20—22 

 Inner gill, 12—18 



Lamellidens — 



Outer gill, 10—20 

 Inner gill, 12 — 15 



Physunio — 



P. ferrugineus — 



Outer gill, 25—28. 



Inner gill, 14 — 16. 

 P. micropteroides — ■ 



Outer gill, 20. 



Inner gill, 15 — 16. 



Lateral tube. 



Only outer lateral tube. 

 Only inner lateral tube. 



"^ Outer tube much more deve- 

 3 lo23ed than the inner tube. 



V. Alimentary Canal. 



(«) The mouth is a transverse slit-like aperture bounded in front and 

 behind by anterior and posterior lips continuous with the outer and 

 inner labial palps respectively. 



(b) Oesophagus. — The course of the oesophagus is different in the three 

 forms : in Lamellidens it is slightly curved or broadly S-shaped, passing 

 obliquely upwards and backwards through the digestive glands. In 

 Solenaia it consists of a short nearly vertical portion (wide in the middle 

 and narrow at both ends) and an elongated horizontal portion conti- 

 nuous behind with the stomach. In Physunio it consists of a vertical 

 portion lying behind the anterior adductor muscle and a short horizontal 

 portion (half as long as the vertical portion) continuous with the 

 stomach behind. 



(c) The stomach is surrounded by a digestive gland ; its cavity is 

 very irregular with folds and furrows which are fairly constant in ar- 

 rangement. 



Behind the opening of the oesophagus and in the floor of the stomach 

 is a transverse fold (tongue-like in Lamellidens, but rather conical and 

 papilla-like in Solenaia and Physunio), with a furrow behind ; the fold is 

 directed upwards and backwards from below and produced laterally 

 to the lateral walls of the stomach. In Solenaia and Physunio there is 

 another fold in front of the conical elevation at the junction of the 

 oesophagus and stomach, which in Solenaia extends backwards and up- 



