RETINA OF LATERAL EYES OF SPHENODON PUNCTATtJS. 317 
Sphenodon, in which, however, I have found no reason to 
doubt the correctness of the older interpretation of the 
membraua limitans externa as being formed from the Muller’s 
fibres. 
b. Detailed Account of the Sense-Cells. 
Sphenodon appears to form no exception to the rule that cones 
only are present, to the exclusion of rods, in the reptilian 
retina. It is generally admitted that oil-globules do not occur 
in rods, but they are present in nearly all the visual cells in 
Sphenodon. The only ones iu which there are no oil-globules 
are the larger members of the double or twin cones — i.e. the 
so-called near cones — and these have other distinctive 
characteristics which mark them out as cones, viz. their 
association with the smaller members to form the well known 
pairs, and their very characteristic shape. There are three 
very definite kinds of cones in Sphenodon, viz. the smaller 
and larger single cones and the double cones. The two 
members of the latter, though always associated, have so 
many differences that they may be conveniently considered 
separately. I shall classify the cones, then, as follows : 
(1) Ordinary single cones; (2) small single cones; (3) 
double cones — (a) near cones, (b) far cones. 
In the fovea centralis only cones of the first type are 
present, densely packed together, and somewhat longer and 
more slender than in other parts of the retina. Iu the 
macula lutea, outside the fovea, a few double cones are 
present also, and the single ones are less closely packed, the 
number in a given space being only about half what it is in 
the central fovea. Over the main part of the retina (fig. 5), 
many more double cones are present, though they are still 
less numerous than the single ones, there being only about 
one double cone to every two ordinary single ones. Ordinary 
single cones only are present towards the ora serrata, where 
they gradually become shorter and thicker and finally 
disappear altogether. 
