674 MAMMALS 



of the muscle in Dasyurus and Marmosa, and in the former genus they 

 lie toward the scleral side of the muscle. In Didelphis the meridional and 

 circular fibers are intermingled in small bundles. The anterior tendon in 

 these three genera, and probably in many others, is formed by a small 

 mass of unpigmented uveal 'meshwork' tissue which lies against the 

 anteriormost part of the sclera and extends forward beyond the iris root, 

 where it tapers to meet the edge of the membrane of Descemet. Between 

 this tissue and the sclera lies the endothelial canal of Schlemm. These 

 relationships are essentially those of the human eye. Within the mar- 

 supial group, then, the ciliary muscle may be situated either as it is in 

 reptiles, or as it is in placental mammals. The transition seems to be 

 made simply by the creation of the meshwork 'tendon', dropping the 

 anterior end of the muscle farther back from the limbus corneas. 



The lens is always relatively large; and in the smaller, more strongly 

 nocturnal types it may nearly fill the globe — as in many small-eyed noc- 

 turnal placentals, e.g. Mus. The flatter lenses occur, as would be ex- 

 pected, in the large-eyed arhythmic forms (Fig. 196a; compare Fig. 

 71 [opossum], p. 173). Even in the flattest lenses there are never more 

 than traces of the reptilian ringwulst, and the lens is always quite out 

 of contact with the ciliary processes. The flatness index of the lens may 

 be little more than 1.0, or as high (in kangaroos) as 1.5. Some sample 

 values follow: 



Horiz. 4> Lens Lens Thick- Index 



Species of eye (D) ness (T) (D/T) 



Marmosa mexicana 



(a mouse opossum) 6.3mm. 4.7mm. 4.5mjTi. L05 



Didelphis virginiana 



(the common opossum) U.O 7.3 6.0 1.22 



Dendrolagus bennetti 



(a tree kangaroo) 15.0 8.3 6.7 1.24 



Osphranter (robustus ?); 



(a rock dwelling kangaroo) 13.0 10.0 1.30 



Macropus giganteus 



(a ground kangaroo) 27.0 13.0 10.0 1.30 



The Marsupial Retina — Through its loss of all of the accessory 

 structures involved in sauropsidan accommodation (except the ciliary 

 processes — and these no longer bear upon the lens), the marsupial eye 

 as a whole is thoroughly mammalian — i.e., placentalian. The retina, 



