240 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



I. Genera with compound eyes having : 



(a) prismatic plano-convex cornea facets; 



(b) round or bi-convex transversely elongate lenses; 

 II. Genera with aggregate eyes of bi-convex lenses ; and 



III. Genera with isolated eyes, one or several stemmata at the extremity of a 

 straight facial ridge. 



He says [1901, p. 27] of the statement of Packard: "This state- 

 ment is altogether wrong, and as I hope to show the trilobites have 

 had eyes entirely different from those of Linmhis, and instead agree 

 with those of the Isopoda and perhaps also with a few other 

 Crustacea." 



At the time Dr. Lindstrom wrote he was unacquainted with the 

 visual surface of the eye of Olcnelliis and contended [1901, p. 9] 

 that all Cambrian trilobites were blind in not having eyes on the 

 upper surface of the cephalon. He thought that they might have 

 been provided with visual organs on the hypostoma. The dis- 

 covery of the faceted surface of the eye of Olenellns glJhcrtl clearly 

 negatives the broad conclusion of the absence of a visual organ on 

 the upper surface of the cephalon and indicates that with sufficiently 

 well-preserved specimens the visual surface will be found on all 

 Cambrian trilobites with eyes. That it has not been found long ago 

 is probably due to the fact that the roughened visual surface without 

 a corneal covering adheres to the matrix and is broken off with it. 

 I do not wish to assert that the eyes of Olenclhis and LUnuJus are 

 constructed on the same plan, but I do think that the outward ap- 

 pearance of the surface of the eye of the young specimens shows 

 that they were similar [pi. 43, figs. 4 and 5]. 



Dr. Lindstrom [1901, p. 71] found macula; on the hypostoma of 

 136 species of 39 genera of trilobites, on 36 species of which it was 

 possible to study the structure of the maculae through sections. He 

 states that while the structure that often characterizes the maculse as 

 a visual organ is very rudimentary, there is no doubt that it sub- 

 served the purpose of the visual organ, even where there is no trace 

 of any definite structure that is preserved in the fossil state. In final 

 conclusion he says [1901, p. 74] : 



We find the maculae of the trilobites present from the oldest Cambrian times 

 and we find also in them a progressive evolution, in some to a high degree, 

 lenses and facets, perfectly identical with those of the eyes on the head shield, 



converting them into true eyes But there are, no doubt, still more facts to 



adduce for filling up extant lacuna; in the knowledge of these matters. 



