MEDIAN EYES IN EXTINCT VERTEBRATES 337 



Diplopterus, showed a pineal foramen in the same position as in the skulls 

 of the Stegocephala (Fig. 170, Chap. 19, p. 237). Some of these also 

 resemble the Stegocephala in the possession of a ring of sclerotic plates 

 around the lateral eyes. The skull of one of the living representatives 

 of the order — Polypterus — shows no pineal foramen, but there is a well- 

 marked pineal diverticulum in the embryo. Moreover, in the nearly 

 related order Chondrostei — including Acipenser (sturgeon) (Figs. 147, 148, 

 Chap. 18, p. 211) and Polyodon (spoonbill) (Fig. 235) — and in the order 

 Holostei — which includes Lepidosteus (bony pike) and Amia calva (bow 

 fin) — the pineal organ, although vestigial, is easily recognizable in the 

 adult animal, and in Amia shows evidence of bilaterality (Fig. 149, Chap. 

 18, p. 213). 



The position and relations of the pineal organ and foramen in living 

 Crossopterygii and allied living orders, when compared with the extinct 

 Osteolepis, appear to confirm the connection of the Crossopterygian 

 fishes with the more ancient type of Osteichthyes, which on account of 

 other general resemblances is considered to be close to the parent stock 

 from which the Stegocephala and modern Amphibia have arisen. This 

 is a point of very considerable interest, since it is in some of the Stego- 

 cephala and extinct reptiles of the Carboniferous, Permian, and Mesozoic 

 periods that the pineal organ seems to have reached its greatest size and 

 possibly highest degree of differentiation. 



Among extinct Amphibia, the parietal foramen is found to be of very 

 variable size. It is, relatively to the size of the skull, very large in Protriton, 

 one of the smaller Stegocephala (Fig. 170, Chap. 19, p. 237). In this 

 primitive animal the pineal foramen lies between the parietal bones in a 

 transverse plane behind the orbital cavities. The lateral eyes were pro- 

 tected by a ring of bony plates in the sclerotic ; these are similar to those 

 in Ichthyosaurus, and they probably indicate that either Protriton itself or 

 its ancestors were able to dive to great depths in the water, and that they 

 served to resist the pressure of the water on the eyeballs. In the skulls 

 of Branchiosaurus ambly stoma and Metanerpeton (Fig. 169, A, Chap. 19, 

 p. 236), which represent primitive examples of the Stegocephala, the 

 pineal foramen was also large. 



The pineal foramen was, relatively to the size of the skull, of moderate 

 size in Eryops megacephalus and in the curious snake-like Dolichosoma 

 longissimum (Fig. 169, B, Chap. 19, p. 236), found in the Permian strata 

 of Bohemia. In the skull of this animal the frontals and parietals are 

 fused into a single elongated plate near the hinder end of which the cir- 

 cular pineal foramen is conspicuous owing to its margins being slightly 

 raised above the general surface of the skull. 



In Diplocaulus magnicornis (Fig. 168, Chap. 19, p. 235) — Permian of 



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