VARIATION IN THE EYES OF THE SILURIAN TRILOBITES EOPHACOPS AND ACASTEAND ITS SIGNIFICANCE

Among the compound eyes of trilobites, the most remarkable are the Sch izochroal type of the suborder Phacopina. As well as representing an a ncient visual system of probably unique kind, schizochroal eyes show p atterns of variation in lens distribution which have figured in discus sions of possible dimorphism and polymorphism in trilobites species, a nd have been used by some authors as taxonomic characters. Eophacops m usheni is a common species in the British Wenlock, and variation in th e lens pattern on the visual surface is described from about 40 well p reserved specimens. Adults typically have 19 or 20 files of lenses, an d two cases are described of individuals with 20 files in the left eye and 19 in the right. Comparable cases occur in Acaste. Dimorphism in the visual surface of A. downingiae is doubtful. The new data on the v ariation of the visual surfaces of E. musheni and A. inflata indicate that visual surface morphology provides a reliable guide to species id entity only in some cases. They also allow Clarkson's developmental mo del for the visual surface to be extended to imply that the initial le ngth of the section of the generative zone actively producing lenses w as variable, and that lens emplacement was initiated at different time s relative to the descent of the generative zone in different individu als. If development of a lens was controlled by the distance from adja cent lens centres, and given that lenses are round and that emplacemen t began in a single horizontal row, hexagonal close packing and the de velopment of dorso-ventral files result automatically. Cubic close pac king could be produced by modifying the spacing factor in successive h orizontal rows. The number of dorso-ventral files of lenses and their relative height are controlled by the length of the active section of the generative zone and its pattern of growth. The existence of indivi duals with eyes differing in the number of files demonstrates that fil e number is a consequence of a developmental programme, rather than be ing under immediate genetic control. Variation in the timing of termin ation of lens emplacement accounts for the observed variation posterio rly and near the base of the visual surface.