588 HIGHER FISHES 



possible once, it may have been possible many times, and secondary 

 derivation from twins may thus account for all teleostean 'doubles'. 



The visual cells are nearly always arranged in a neat mosaic; and 

 where this is true, the unit of the mosaic is almost invariably a perfect 

 square, with a twin cone on each side (Fig. 170a).* In some instances, 

 where the visual cells are particularly large {e.g., Fig. 170c), the mosaic 

 is visible ophthalmoscopically in the living animal Where the rods are 

 very small and very numerous, as they usually are, they often occur in 

 clusters or bouquets, with their myoids of unequal length so that the rod 

 mass is pseudostratified — there being no room for the rods all to be 

 brought into a single plane even in either extreme dark- or extreme 

 light- adaptation. In two families — the elephant-fishes (Mormyridas) and 

 the ten-pounders (Elopids) — both rods and cones are gathered together 

 into great bunches, each surrounded by the heavy conical processes of a 

 circle of adjacent pigment-epithelial cells. In many teleosts, the cone 

 nuclei lie partly or wholly through the external limiting membrane, and 

 are much larger and less stainable than those of the rods (c/. Fig. 94, 

 p. 237). The foot-pieces are then very different, those of the cones being 

 heavy and dendritic while those of the rods are filamentous and termi- 

 nate in tiny smooth end-knobs. These differentiations of nuclei and foot- 

 pieces do not occur below the teleosts; nor do they appear on the land- 

 animal side of the fence until the amniotes are reached. Though the 

 physiological meaning of these differentiations is obscure, the sharing of 

 them by the teleosts, birds, and placental mammals seems definitely 

 correlated with the presence, in these same groups, of species having 

 such things as extensive accommodation, high visual acuity, brief biolog- 

 ical moments, fovea, and color vision. The teleostean eye and retina, 

 at their best, are outstanding in 'perfection' among all the fishes, and 

 represent the fishes' nearest approach to the ocular quality of the very 

 highest vertebrates. 



(D) Cladistians and Dipnoans 



These are the living 'lunged' fishes — though by no means the only 

 ones which ever use the swim-bladder for breathing air at the surface. 

 The two living cladistian genera, Polypterus and Calamoichthys (both 

 inhabiting African rivers), were formerly classed as crossopterygians, 



^Obviously, it would be decidedly worthwhile to make tangential sertions of the retin* of 

 Amid and some of the Gadidae; for if the conjugate elements of these forms are found to 

 be arranged also in squares, our ideas about the origin of twin cones may be clarified. 



