228 



TRILOBITA 



CHAP, 



111 



Three types of compound eye have been recognised 

 Trilobites 1 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 



FIG. 139. Eyes of Trilobites. (After Lindstrom. ) A, B, Sphaerophthalmus alatus, Aug. 

 Upper Cambrian. Vertical and horizontal sections, x 100. C, Asaphtis fall-ax, 

 Dalm. Horizontal section, x 60. D, Nileus armadillo, Dalm. Vertical section, 

 x 60, a, prismatic lenses ; b, cuticle ; c, part of free cheek. E, Dalmanites 

 vulgaris. Salt. Part of eye, x 30. F, Dalmanites imbricalulus, 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, 

 x60. 



Examples of this are seen in 



continuous over the entire eye. 

 Bronteus and Sphaerophthalmus. 



1 Lindstrom, " Visual Organs of Trilobites," Svenska Vet. Akail. Hnndl. xxxiv., 

 1903. Exner, Physiol. d. facet t. Ait-yew v. Krcbsen u. Insccteii, 1891, p. 34, pi. ii. 

 iigs. 18, 19. 



