40 Proceedings of the Royal Physical Society. 
there seems to me no sufficient reason to doubt that the ridges were originally 
strictly longitudinal in position. 
The great importance of the Dipnoan heart as an aid to the comprehension 
of the morphology of the heart of the tetrapod Vertebrates has long been 
recognised, ¢.g. by Boas1—perhaps the most clear-sighted worker in this 
department of morphology. Here, in addition to Boas’ results, I shall make 
use of the excellent study of the structure and development of the heart of 
Fic. A. Fie. B. 
A. View of a piece of flexible tubing bent so as to illustrate the flexure of the cardiac 
tube in Dipnoi, The part of the figure above the horizontal line represents the conus as 
seen from the ventral side. The original dorsal (D), ventral (V), right (R), and left (L) 
surfaces are indicated by longitudinal lines. 
The part of the figure below the horizontal line represents the hypothetical similar 
curvature of the atrioventricular portion of the primitive cardiac tube. 
B. Represents the portion of tube above the horizontal line in Fig. A after it has been 
straightened out so as to have a right-handed spiral twisting distributed throughout its 
length. 
Lepidosiren which has recently been carried out by Dr Jane Robertson.” The 
first important point about the Dipnoan conus is that in Ceratodus one (the 
originally right-hand ridge), and in Lepidosiren and Protopterus two (the 
originally right-hand, and also the originally left-hand), of the longitudinal 
rows of pocket-valves have reverted towards the ancestral condition of a 
continuous ridge projecting into the lumen of the conus. This is one of those 
cases constantly turning up in embryology and comparative anatomy which 
1 1880, Morph. Jahro., Bd. 6. 
2 1913, Quart. J. Mier. Sci., N.S., vol. lix, 
