258 CLARKE. [VoL. II. 
Between 30 and 40 mm. is 62.5 
Vee and’ 45 ina. s G2r6 
From 40 mm. upwards, 58 
The calculated average basal cephalic width in this species, 
deduced from measurement of 1518 cephala, is 22.8mm. The 
material from which this average is derived was unselected, 
much of it collected without reference to quality or size, and is 
fairly representative. I therefore venture the statement that 
the average Phacops rana has a width across the posterior 
margin of the cephalon of approximately 22.8 mm. 
It is, moreover, probable that 22.8 mm. is approximately the 
dimensional index for the average normal adult of this species. 
In all specimens of the entire animal which have passed under 
observation, varying in axial length from Io mm. to 100 mm., no 
evidence has appeared of any developmental change in the suc- 
cessive stages of growth, except in the increase and diminution 
of the number of corneal lenses. Save in this one respect the 
species assumed all the features of maturity at a very early 
point in its history; and the data given above conclusively 
indicate that in this feature, also, maturity was attained with 
this stage of growth. The important conclusion here drawn is 
that the number of lenses increases from youth to maturity (di- 
mensional index approximately 22.8 mm.), axd decreases from 
maturity to senility. 
Two questions immediately arise from this inference; (2) how 
is the number of lenses increased? and (4) how is it diminished ? 
These points will be adverted to in a following section. 
STRUCTURE OF THE LENS. 
Sections across the visual surface show that the lenses are 
unequally bi-convex, the curvature being greatest on the proxi- 
mal surface. This inferior surface is perforated by a central 
circular aperture. Vertical sections of the lens when favorably 
preserved, also show this envelope as a simple, thin, distinctly 
black or brown corneous film; and in natural casts of the inter- 
nal surface of the visual area, the ommatidial cavities are repre- 
sented by a series of shallow cups standing on short pillars, and 
each bearing at its centre a little ball, which is the filling of the 
interior of the lens. These lenses are consequently corneal and 
