N. grandis 1,785 fms. Eye 1⁄11th length of the head N. macrops 375 fms. Eye 2⁄9 length of the head N. ocellatus 350 fms. Eye ¼ length of the head N. gillii 111 fms. Eye 1⁄(3⅔) length of the head Similarly in the species of the widely distributed deep-sea genus _Macrurus_: the species _M. parallelus_, _M. japonicus_, _M. fasciatus_, &c., usually living in water less than 1,000 fathoms deep, have large and in some cases very large (_M. fasciatus_) eyes, but _Macrurus filicauda_, _M. fernandezianus_, _M. liocephalus_, _M. Murrayi_, _M. armatus_ have small eyes. Some deep-sea fish have their eyes reduced to a mere rudiment; such as _Ceratias uranoscopus_, _C. carunculatus_, _Melanocetus Murrayi_, _Typhlonus nasus_, and _Aphyonus gelatinosus_, but not even a rudiment of an eye is to be found in _Ipnops Murrayi_. But the fish of the greatest depths are by no means always characterised by small eyes. _Malacosteus_, a typical deep-sea form, has very large eyes, and so have _Bathylagus_, living in the enormous depth of 3,000 fathoms, and _Bathytroctes_, in 1,090 and 2,150 fathoms. The result of recent deep-sea work, then, has been to show that as we proceed from shallow shore water to depths of 500 to 900 fathoms the eyes of the fish become larger, but in greater depths than 1,000 fathoms the eyes of some fish become considerably reduced, but those of others become still more enlarged. In the greatest depths of the ocean in fact it seems very probable that nearly all the fish are characterised by either very large eyes or very small ones. We cannot expect to learn very much at present from the study of the eyes of deep-sea mollusca. The Cephalopods form the only class of this Phylum whose genera invariably possess large and well-developed eyes, and there does not seem to be any very marked increase or decrease in the size of the eyes of the few deep-sea cuttlefish that are known to us. The eye of _Nautilus_ is certainly remarkably interesting, but as this genus is the only representative of its order, and is known at times to float upon the surface of the ocean, it would certainly be erroneous to attribute the peculiarity of the structure of its eye to its ‘temporary’ deep-sea habits. We are still ignorant of the usual habitat of the remarkable genus _Spirula_, notwithstanding the fact that many of the tropical beaches are very largely composed of its empty shells. Whether it is a deep-sea dweller or not, we know nothing at present of the character of its eye, so that it can throw no light upon the problems we are now discussing.