MONOTREME RETINA; MARSUPIAL EYE 671 



echidnas, and the total absence of cones,* implies a stricter nocturnality 

 than that of the duck-bill; but no great difference in habits seems to 

 have been noted. 



The types of visual cells are direct derivatives of those of the Saur- 

 opsida (Fig. 195a, b; cf. Figs. 176b, 177a, 193b, pp. 612, 615, 660; and 

 see Plate I) . In Ornithorhynchus the single and double cones have lost 

 the paraboloid but have retained the oil-droplet, which was very recently 

 found to be colorless. The rod and cone nuclei are not differentiated, but 

 are both 'cone-like' as in all sauropsidans excepting nocturnal birds. In 

 Tachyglossus the cones themselves have gone. The complete monotreme 

 visual-cell pattern (of Ornithorhynchus) fits equally well the accepted 

 idea that the monotremes are a lateral branch of the stock which cul- 

 minated in the marsupials, and the minority notion that the monotremes 

 evolved independently from reptiles. The simplification of the cones in 

 the duck-bill, and their discard in the echidnas, are natural consequences 

 of adaptation for dim-light activity. 



The Marsupial Eye — Marsupials have a nictitating membrane, but it 

 is never highly developed. Its gland (the Harderian) is present, along 

 with the lacrimal. A retractor bulbi is present; but no details are on 

 record concerning the extra-ocular muscles. 



The eyeball is perfectly spherical in a very few species and is prac- 

 tically spherical in all others. The horizontal and vertical diameters are 

 always equal, and usually exceed the axial length (by up to 10%). This 

 relationship is reversed in some opossums. The topography of a sagittal 

 section is always like that in nocturnal and arhythmic placentals (Fig. 

 196a; cf. Fig. 71, p. 173). The diameter of the cornea is always great 

 in proportion to the diameter of the eyeball — 66-80% in kangaroos, 82% 

 and 87% in opossums (Didelphis virginiana and Marmosa mexicana 

 respectively), 91% in the cuscus {Trichosurus vulpecula). The cornea 

 is horizontally ovoid only in large kangaroos, in simulation of their 

 ungulate counterparts. 



The sclera is fibrous, entirely devoid of cartilage (except for some 

 questionable nodules in the marsupial 'golden mole', Notoryctes) . It has 

 thus taken the final step in the elimination of the cartilage-and-bone 

 system of the reptilian eyeball wall, and the basically spherical form of 

 the marsupial eyeball is the expression of this elimination {cf. snakes). 



♦Certain in the case of Tachyglossus; probable in Z.<iglossus, but Kolmer's material was too 

 badly histolized to make possible any study of the visual cells. 



