404 Dr. T. Williams on the Mechanism of Aquatic 



(1) to be rendered quite sure that R. ccenosus is the plant that 

 now inhabits those ditches, and (2) that it was the true R. hede- 

 raceus alone that grew there formerly. I have most frequently 

 found R. coenosus in rather elevated situations, where no source 

 of artificial heat could affect it. 



12. R. hederaceus (Linn.) ; leaves all roundish reniform with 

 3-5 shallow rounded lobes vndening to their base, petals 

 scarcely exceeding the calyx, style prolonging the inner edge of 

 the ovary, carpels ^-oval or ^-obovate with a lateral point. 



R. hederaceus, Linn. Sp. PL 781 ; Eng. Bat. t. 2003; Reichenb. 

 Icon. Fl. Germ. iii. Ran. t. 2. 



Stem floating or creeping upon mud, branched, nearly round. 

 Leaves usually spotted; lobes separated by shallow notches, 

 widening gradually from their base to a narrow rounded end, 

 often broadly triangular, entire or rarely with a slight notch at 

 the top. Petioles long, semicylindrical. Stipules long, much 

 adnate, blunt, denticulate. Peduncles not narrowed upwards, 

 much falling short of the leaves. Flowers very small. Petals 

 about equalling or a little exceeding the calyx, narrow, 3-nerved. 

 Stamens 6-8. Stigma short, oblong. Receptacle spherical, 

 naked. Carpels compressed below, blunt and inflated above, 

 inner edge nearly straight, laterally tipped with the style or 

 pointless. 



Flowering from June to September. 



This plant is probably generally distributed, but as R. ccenosus 

 is often mistaken for it, I may mention that I know of its ex- 

 istence at Inverarnan at the head of Loch Lomond, near Llan- 

 beris in Caernarvonshire, Lanwarne in Herefordshire, Needwood 

 Forest in Staffordshire, Tiptree Heath in Essex, Triplow and 

 other places in Cambridgeshire, near Haverfordwest in Pem- 

 brokeshire, Ninham in the Isle of Wight, and Bovey Heathfield 

 in Devonshire. 



XXXIV. — On the Mechanism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

 By Thomas Williams, M.D. Lond., F.L.S., Physician to the 

 Swansea Infirmary. 



[With a Plate.] 



[Continued from p. 329.] 



General and Minute Anatomy of Branchial Organs in the 

 Gasteropod Mollusks, 



The author is not acquainted with any English or continental 

 researches on the subject of the present paper. While the 



