228 TRILOBITA CHAP, 
Three types of compound eye have been recognised in 
Trilobites '—holochroal, prismatic, and schizochroal. 
1. In the holochroal eye (Fig. 139, A, B) the lenses are 
globular or biconvex and close together, so that the cornea is 
G 
Fie. 139.—Eyes of Trilobites. (After Lindstrém.) A, B, Sphaerophthalmus alatus, Ang. 
Upper Cambrian. Vertical and horizontal sections, x 100. ©, Asaphus fallax, 
Dalm. Horizontal section, x 60. D, Nilews armadillo, Dalm. Vertical section, 
x 60, a, prismatic lenses; 6, cuticle; c, part of free cheek. E, Dalmanites 
vulgaris, Salt. Part of eye, x 30. F, Dalmanites imbricatulus, Ang. Vertical 
section of eye, with a part of the free cheek on the left, x 60. G, H, Harpes 
vittatus, Barr. G, The two lenses of one eye, x 8 ; H, vertical section of the same, 
x 60. 
continuous over the entire eye. Examples of this are seen in 
Bronteus and Sphaerophthalmus. 
1 Lindstrom, ‘f Visual Organs of Trilobites,” Svenska Vet. Akad. Handi. xxxiv., 
1905. Exner, Physiol. d. facett. Augen v. Krebsen uw. Insecten, 1891, p. 34, pl. ii- 
figs. 18, 19. 
