THE MONOTREME EYE 669 



absence of a structure — occurs in some living reptilian group. The ciliary 

 web alone* thus keeps the eye of the monotreme from being entirely 

 reptilian, with its closest morphological resemblance to the eye of the 

 likewise-nocturnal crocodilian. 



The lens is unexpectedly small, flat, and anterior in position. The 

 topography of the monotreme anterior segment, particularly in the 

 echidnas, is in fact not at all sauropsidan but more like that of the 

 sirenians and primates. Tachyglossus has the flattest of all lenses, with 

 a flatness-index (diameter divided by thickness) of 2.75.+ This value is 

 closely approximated elsewhere only in some of the higher primates, 

 including man (ca. 2.7). At its equator, the lens epithelium is twice 

 as tall as at the anterior pole, constituting perhaps a vestigial ringwulst. 

 A similar situation obtains in the duck-bill, and also in some marsupials. 

 The lens of the duck-bill, in keeping with the aquatic habit, is much less 

 flat— 2.66/1.93 = 1.38 (Kahmann), 2.45/1-75 = 1.4 (Gunn), or 1.5 

 (from a photograph of O'Day's — scale not given) . O'Day compares its 

 form with that of the lens of the local Murray turtle, Chelodina longi- 

 collis. 



No monotreme has any demonstrable accommodation, and there are 

 no reports as to refractive conditions. It is not known whether Ornitho- 

 rbynchus approaches emmetropia in either air or water, but the impli- 

 cations are that the eye is better adjusted to the latter medium. The 

 echidna eye looks as though it must be extremely hypermetropic; but 

 only a study of the living animal can settle the matter. 



In both Ornithorhynchus and Tachyglossus the numerous zonule 

 fibers arise from the coronal zone of the ciliary body and from the free 

 portions of the ciliary web (including its very edge), and insert com- 

 pactly on the extreme periphery of the lens, largely just in front of its 

 equator. 



The Monotreme Retina — The rather thin sensory retina extends 

 farther forward temporally than nasally in Ornithorhynchus (but not 

 in Tachyglossus?) , suggesting an importance of the binocular field. 



*And the unstriated condition of the sphinaer pupillcc; but there is no reason to think that 

 this is a new muscle. Iris muscles have been independently evolved several times of course; 

 but the mammalian sphincter has, in all probability, been inherited directly from the rep- 

 tiles. Not so the mammalian dilatator. 



fMeasured in O'Day's preparations (3.3mm / 1.2mm.); Franz gives 3.0/0.8 = 3.7, but 

 expresses doubt as to the validity of these figures. Kolmer gives 2.88/. 96 = 3.0 for 

 Z^dglossus, but his material was preserved many hours post mortem. 



