Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells.
(b) Bivalvia (= Conchae): Mya, Solen, Tellina, Cardium, Mactra, Donax, Venus, Spondylus, Chama, Arca, Ostrea, Anomia, Mytilus, Pinna.
In the Coralline Limestone the following fossils have been noted :- Spondylus, Ostrea, Pecten, Cytherea, Arca, Terebratula, Orthis, Clavagella, Echinus, Cidaris, Nucleolites, Brissus, Spatangus; in the Marl the Nautilus zigzag; in the Yellow, Black and Greensand shells of Lenticulites complanatus, teeth and vertebrae of Squalidae and Cetacea; in the Sandstone Vaginula depressa, Crystallaria, Nodosaria, Brissus, Nucleolites, Pecten burdigallensis, Scalaria, Scutella subrotunda, Spatangus, Nautilus, Ostrea navicularis and Pecten cristatus (see Captain Spratt's work and papers by Lord Ducie and Dr Adams).
In some Lamellibranchs (Pecten, Spondylus, Pholas, Mactra, Tellina, Pectunculus, Galeomma, &c.), although cephalic eyes are generally absent, special eyes are developed on the free margin of the mantle-skirt, apparently by the modification of tentacles commonly found there.
Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Pinna, Pectunculus, Modiola, Cardium, Tellina, Mactra, Venus, Solen, Pholas and Galeomma.
In Pecten and Spondylus, however, they have been fully studied (see fig.
Spondylus; shell with spiny ribs, adherent by the spines.