mie STRUCTURE. AND. DEVELOPMENT OF) THE 
VISUAL. AREA IN THE TRILOBITE, 
PHACOPS RANA, GREEN. 
By JOHN M. CLARKE. 
To students of the fossil Arthropoda it should be a matter 
of congratulation that so great success has been achieved by 
paleontologists in solving the problem of anatomy and develop- 
ment in the extinct crustacean order, the Trilobita. Though 
much may remain to be done, too great esteem cannot be 
accorded to those who have contributed to what has been ac- 
complished ; Eichwald, Burmeister, Volborth, Quenstedt, Rich- 
ter,! Barrande, Hall, Billings, Ford, Walcott, Woodward, Mickle- 
borough, Matthew, Packard. To the labors of these men is due 
our knowledge of the development of the trilobite from the 
ovum to maturity; of its delicate locomotive and respiratory 
apparatus ; and somewhat of its reproductive, alimentary, and 
muscular anatomy. 
The present paper endeavors to throw some light upon the 
structure and development of the eye in atypical representative 
of an extensive group of trilobites, Phacops rana, Green. In 
the study of this organ abundant material consisting of several 
thousand specimens has been accessible, in the majority of in- 
stances only those being utilizable in which the lenses of the eye 
1 It may be well to call attention to the fact that Richter’s single observation upon 
the ventral anatomy of Phacofs, which has been overlooked by later investigators, is 
of much greater significance than the author himself accorded it. In the Beitrag zur 
Paléontologie des Thiiringer Waldes, 1848, Pl. 2, Fig. 32, is given an enlarged view 
of a transverse section through one-half the thorax of a De- 
vonian Phacops (species not given). As this work may not ley oi x 
be generally accessible, the figure is here reproduced; and - 
although the author states (of. cit. p. 20) that the section 
serves to establish Burmeister’s conception of the ventral anatomy of the trilobite, 
in the light of Walcott’s demonstration of the spiral branchiz in Calymene senaria 
it appears that what is represented here is a section of one of these appendages. I 
may add that I have also detected evidence of these spiral branchiz in Phacops rana, 
