270 CLARKE. (Vou. He 
two vertical rows at the left, are two casts of immature lenses, in one of which the 
filling of the corneal cavity is just discernible; in the other the ommatidial cavity 
appears not to have penetrated to the upper surface. Several features of similar 
character are to be seen on parts of the specimen not represented in the figure. The 
portion of the eye represented is the posterior one-third of the right eye. 
Fic. 26. Two cavity-fillings from the same specimen enlarged to 15 diameters, 
showing the small size of the balls compared with the probable size of the entire 
cavity of the cornea. 
Fic. 27. A section through the eye and adjoining parts of Phacops rana, showing 
the lenses and the interlensar sclera. Enlarged to 3 diameters. 
Fic. 28. The same enlarged to 6 diameters, showing the continuity of the porifer- 
ous integument of the head with the interlensar sclera, the double convexity of the 
lenses, and the depth of the ommatidial cavities in the sclera. At the right of the 
first lens in the series is an internal depression on the integument which appears to 
indicate the position of a newly developing ommatidium. No evidence of a lens was 
visible at this point before the section was made. 
Fic. 29. Three lenses with their interstitial integument, from the same specimen, 
enlarged to 10 diameters. There is a slight difference in the character of the matrix 
filling the cavity of the cornea, it being possible to distinguish the line between the 
anterior and posterior divisions of the space. The dark color of the lower portion 
of the sclera is due to an increase of pigment. 
Fic. 30. A portion of the eye of Phacops rana, from which the sclera has been 
removed by natural causes, leaving the cornez standing on pillars of matrix. En- 
larged to 12 diameters. 
Fic. 31. A natural section of the eye of Phacops cristata, var. pipa, enlarged to 8 
diameters. The sclera has been removed, and the doubly convex surface of the len- 
ses is well shown. 
Fic. 32. The eye of Mesothyra Oceani, Hall, enlarged to 3 diameters, showing 
the strong optic node with a simple, deep pit at its summit. The figure is taken from 
the Paleontology of New York, Vol. VIL., Pl. 32, Fig. 2. 
