MEDIAN EYES IN EXTINCT VERTEBRATES 



331 



original simple character or degenerated, and in some cases when dis- 

 placed towards the median plane, by the great development of the lateral 

 eyes, the median pair of eyes fused with each other to form a cyclops 

 eye, as in Tabanus (Fig. 20, Chap. 3, p. 26) and in many of the Crustacea. 

 In the stem which branched off to form the vertebrates it may be supposed 

 that in the evolution of the lateral eyes the simple optic pit which arose as 

 a downgrowth from the medullary plate (Figs. 5 and 6) became trans- 

 formed into a stalked vesicle which afterwards came in contact with the 

 surface layer of epithelium, as occurs in the ontogenetic development of the 

 typical lateral eyes of vertebrates, and that this stage was followed or 

 accompanied by inversion of the retina and the development of an epithelial 

 type of lens from the overlying ectoderm. Further, that at an early stage 

 in the evolution of the lateral eyes a cartilaginous or bony capsule was 

 formed around each, and served as a special protective covering corre- 

 sponding to the similar nasal and otic capsules. When we examine the 

 skulls of the earliest fossil fishes, such as the Anaspida of the Silurian 



a. b. \i/ c. 



Fig. 232. — Schematic Sketches of the Cranial Roof of the Norwegian 

 Anaspida, showing the Pineal Plate and Foramen ; the Orbital Sceral 

 Plates and General Arrangement of the Scales. (After Kiaer.) 

 a. : Pterolepis. b. : Pharyngolepis. c. : Rhyncholepis. 



period (Fig. 232) or that of Osteolepis (Fig. 132, Chap. 17, p. 182), a 

 lobe or paddle-finned fish of the Devonian period, we find that the orbital 

 cavities for the lateral eyes, are surrounded by a series of flat plates — in 

 Pterolepis (Fig. 232) six in number on each side — and that there is a 

 single pineal foramen in the centre between the two fused frontal bones. 

 It seems probable, therefore, that the median eyes or their stalks were 

 already fused, or possibly that one member of the pair had become sup- 

 pressed owing to the more active growth of the other member of the pair. 

 The median posterior part of the skull in these fishes (Fig. 132) was 

 separated by an interval or articulation from the anterior part formed by 

 the frontals, post-frontals, post-orbitals, and squamosals. This median 

 part had grown backwards over the hinder part of the brain and formed 

 the parietal, supratemporal, and occipital region of the skull, whereas 



