162 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



of that genus. The outside of the shell is rather rough, and, even 

 without the assistance of a lens, this is seen to be due to a number 

 of little crystalline projections which stud the surface quite 

 irregularly. Under the microscope the upper part of the shell is 

 seen to be covered with an immense number of little ridges on 

 which the eyes are located for the most part, though some are found 

 in the grooves between each ridge. The number of eyes is 

 considerable, and both sides of the shell seem to be endowed alike. 

 On viewing the shell structure by transmitted light, a somewhat 

 broad or widened dark margin is perceived round each of the eyes, 

 forming what may be regarded as the pigment-coat. The pupil 

 itself is not easily seen. In a few specimens it is only slightly 

 darker than the surrounding crystalline lens ; but by transmitted 

 light it appears in nearly all. The eyes so thickly cover the surface 

 of the shell that it looks mottled and clouded. Some of them are 

 of large size, and the pupil very large. This shell ought to be 

 one of the best for making observations upon, and it certainly is a 

 very good instance of these newly discovered optical arrangements, 

 from the wonderful numbers of the eyes on the shell-surface. The 

 nerve structure of the shell has only been partially examined. 

 There are two or three other species in Australia, all with the 

 same peculiarities. 



But I come now to a species which, if I am right in what I 

 suggest as to the meaning of its structure, is certainly the most 

 wonderful instance that has come under my notice, while it gives 

 us a clue to the way in which we must regard these multitudes of 

 ocular organs. Trigonia it will be remembered, is one of these 

 remarkable genera of shells, which help to give the fauna of 

 Australia so peculiar and ancient a character. The Trigonise are 

 principally Mesozoic, being especially abundant and characteristic 

 fossils of the secondary deposits from the Lias to the Cretaceous. 

 Two or three species occur in the Australian tertiary rocks ; but 

 there are four living species in our seas. The shell is trigonal 

 with tubercles and radiating ribs, while the two long lamellar 

 teeth in one valve and their sockets in another have conspicuous 

 vertical grooves. The interior of the shell is brilliantly nacreous 

 and most delicately coloured. If the surface-structure of Trigonia 

 lamarckii, Gray, be examined with a lens, it will be noticed that 

 the whole is shagreened with minute glassy tubercles, lying close 

 together, like a pavement of symmetrical mosaic work and 

 forming one of the principal beauties of the species. Under the 

 microscope it is seen that the shagreened appearance is due to the 

 surface of the valve being divided into a number of hexagonal 

 facets, in the centre of which there is what Dr. Woodward and 

 other observers regarded as a nucleus, but which is seen, by 

 powerful magnifiers, to be due to a curiously formed projection 



