336 THE PINEAL ORGAN 



which the last survivors retreated at the end of the Mesozoic era." In 

 the extinct Dipnoi (Dipterus, Ctenodus, Sagenodus) found in the Old 

 Red Sandstone (Devonian) the general form of the animal resembles the 

 primitive fish Osteolepis (Fig. 132, Chap. 17, p. 183), and the roof of the 

 skull consists of numerous small, flat, dermal bones most of which are 

 paired. No parietal foramen was, however, present in the extinct Dipnoi, 

 nor is a pineal foramen present in Ceratodus, the most primitive of the 

 living Dipnoi (Fig. 156, Chap. 18, p. 222), and in this animal it will be 

 seen that extensive fusion of the bones of the skull has taken place, only 

 two elongated bones being present in the median region of the roof of the 



Fig. 236. — Dorsal View of Head of Dipterus, showing Dermal Bones of 

 Roof of Skull and Pores of the Lateral-line System in One of the 

 Earliest Known Fishes. (After Goodrich.) 



Fr. : frontal. Po. : postorbital. 



//. : interfrontal. Poc. : postoccipital. 



It. : intertemporal. So. : supraorbital. 



Pa. : parietal. Soc. : supraoccipital. 



Pf. : prefrontal. Ta. : tabular. 



skull, namely the " ethmoid " — probably including the frontal, interfrontal, 

 and prefrontal elements — and the " occipital," which is formed by the 

 fusion of the parietals with the supraoccipital. The Dipnoi, therefore, 

 although in other respects they constitute a very interesting primitive 

 type of fish, are to be regarded as a degenerating side-branch and not 

 likely to afford any clue as to the origin of the pineal organ, which even 

 in the fossil types belonging to this order did not perforate the skull. 



According to the description given by A. S. Woodward in 1922, the 

 real links between the fishes and amphibians appear to be the paddle- 

 finned fishes — Crossopterygyii — including Polypterus and Calamoichthys, 

 the ancestors of which, e.g. Osteolepis (Fig. 132, Chap. 17, p. 183) and 



