668 MAMMALS 



tionship, with its whole cornea 320[X thick centrally (with 264^, of 

 propria) and 540[1 peripherally (460[X of propria). Comparable figures 

 for Tachyglossus are 350-290, 330-210. 



The chorioid is only 50[X thick in the duck-bill, a little more than 

 twice this thick in Tachyglossus. Histologically, it is ambiguous — as 

 turtle-like as it is 'mammalian'. The pigmented, laminated suprachori- 

 oidal layer or 'lamina fusca' is conspicuous, as is the choriocapillaris, 

 whose elements are unusually large in lumen and are readily seen to be 

 connected with the large veins. 



In all three genera the iris is most simple, its web consisting of little 

 more than the two heavily pigmented retinal layers and a few small 

 blood vessels attached loosely to the anterior face. There is no dilatator, 

 but there is a massive sphincter around which the pigmented retinal 

 layers are rolled so that their mutual edge lies on the anterior face of 

 the iris. The root of the iris lies opposite the limbus in the duck-bill, but 

 well back of this landmark in the echidnas. There is no pectinate liga- 

 ment; but, as in reptiles which lack one, there is a thin anterior contin- 

 uation, past the iris root, of ciliary-body connective tissue, which is 

 adherent to the inner surface of the fibrous tunic and tapers to a knife- 

 edge aligned with the peripheral margins of the Descemet's layers. The 

 canal of Schlemm is embedded in this uveal meshwork tissue, as it is 

 in sauropsidans in general. The iris is dark brown in life, the pupil 

 always circular. 



TThe anterior continuation x»f the chorioid forming the uveal portion 

 of the ciliary body is thin, only lightly pigmented, and not sharply de- 

 marcated from the inner layers of scleral fibers except where it underlies 

 the tallest portions of the ciliary processes. There is no trace of a ciliary 

 muscle, and the writer is quite unable to imagine what it may be that 

 others have mistaken for one. The ciliary processes are low, puffy, and 

 tortuous, and number about 60 in Tachyglossus. Their anterior ends are 

 interconnected by an annular shelf-like structure — like a miniature iris — 

 the 'sims'. This German term has never been translated; perhaps it is 

 high time that it was. Since the sims connects the ciliary processes, which 

 give the ciliary body its name (cilia = hairs or threads) , after the fashion 

 of the webbing which connects the toes of a duck or a frog, it will be 

 called here the 'ciliary web' (Fig. 194b, cw). 



The ciliary web is a decidedly mammalian character, shared by many 

 marsupials and placentals but by no sauropsidans. Every other feature 

 of the monotreme eyeball — whether the feature is a structure, or the 



