262 CLARKE. [Vot. II. 
STRUCTURE OF THE SCLERA. 
The interlensar sclera is continuous with the test, and its 
structure is in all points identical with that of the test. 
The vertical tubules and smaller tubulipores, with which 
nearly every part of the test of Phacops rana is densely perfo- 
rated, are plainly visible in every section of the sclera, no dif- 
ference in the structure of the parts being discernible, although 
the thickness of the sclera is somewhat less than that of the 
adjoining portions of the test; however, the thickness of the 
test is of necessity very variable in different parts of the ani- 
mal. Not infrequently eyes have been observed, preserved as 
casts in decomposed chert, in which the tubules of the sclera 
are represented by delicate rods traversing the vacant space left 
by the removal of the integument. 
ABNORMALITIES in the arrangement of the corneal lenses are 
of comparatively rare occurrence. They appear to be due, in 
every instance observed, to the failure of a lens to develop at 
the proper time and place in its own row, but in no case has a 
lens appeared so out of place as to be intercalated between 
rows. Marked abnormalities, such as that represented in Figs. 
II and 12, are usually confined to one of the two eyes. It is, 
however, not uncommon to find the right and left eye differing 
in the number of lenses in the corresponding rows, either with 
or without affecting the total number of lenses. In the follow- 
ing example the total is the same, although the arrangement 
differs : — 
Right eye. 6. 
6. 
9. 60 
Left eye. O. 60 
8. 
8.1 
9590 716;4. 2 
9.10. 7:,0,\A. 2 
In another example both the number in the corresponding 
rows and the sum total differ :— 
Right eyes 5.7. 8. 7. 77.,0- 42 ee Ay 
Left eyes 0) 5-19.09. 10.9.1 7-..0- 4a 2 — 55 
These instances of irregular development may be due to 
pathologic or other organic conditions of the animal; perhaps, 
also, in part to external influences. 
