PLATE LXVIII. 



Fig. 1. Diagrammatic sketch of the head of Ipnops to show the relation of the various structures. S, crest of 

 cartilaginous median septum of skuU; E, pigmented margin of phosphorescent organ; T, bony longi- 

 tudinal canal ; a, orbital (?) convex area of skull-roof ; h, Hatteued concentrically striate area of 

 same ; d, posterior region of phosphorescent organ lying over the cranial roof ; e, region of phosphor- 

 escent organ lying between the bony longitudinal canal and the septum. 



Fi". 2. Vertical transverse section across the snout of Ipiiojjx, about on a level with the point indicated by 

 the letter T in fig. 1. Lower jaw removed. S, horizontal plate and median septum of hyaline 

 cartilage, shaded dark P, transparent roof of skull ; T, longitudinal canal on surface of skull-roof 

 in section ; M, its contained mucous canal ; N, nasal branch of fifth nerve in section ; 0, plate of 

 connective tissue completing the median septum ; /?, phosphorescent organ ; X, connective tissue 

 layer beneath the organ ; W, section of internal margin of phosphorescent organ ; V, section of 

 external margin of the same. Each margin shows a thickening of pigment. 



Fig. 3. A portion of the phosphorescent organ viewed from above by reflected light. Ten of the hexagonal 

 columns remain in sita. 



Fig. -i. A portion of the same viewed by transmitted light, a, ramifications of capillary blood-vessels. 



Fig. 5. Vertical section of the phosphorescent organ, showing the rounded bases of the hexagonal columns. 

 The under surface is partly seen in perspective ; e, surface layer of cells ; h, rods ; c, hexagonal 

 pigment-cells. 



Fig. 6. Vertical section of phosphorescent organ through three hexagonal columns, \cry highly magnified. 

 a, blood-corpuscles in a small blood-vessel seen in section. The blood-vessel is held in situ by part 

 of the connective tissue layer lying beneath the phosphorescent organ, in which lies a large 

 ramified pigment-cell, h; c, hexagonal pigment-cell forming base of a hexagonal column seen in 

 section; d, its nucleus; c, superficial cells of phosphorescent organ; f, rods; Ji, bracket embracing 

 the rods belonging to a single hexagonal column ; <j, pigmented ramifying string seen in section. 



Fig. 7. Two of the rods with their corresponding cells, very highly magnified. Only about two-thirds of the 

 length of the rods is shown. 



Fig. 8. Two rods, less magnified, with nuclei near their inferior extremities. 



Fig. 9. The cells of the superficial cell-layer of the phosphorescent organ viewed from above. 



Fig. 10. Hexagonal pigment-cells viewed from the under surface of the phosphorescent organ, i^lso one of 

 these from above, showing its cup-like form. 



Fig. 11. One of the hexagonal columns viewed from above, so that the component rods are seen in optical 

 section. 



Fig. 12. View of the phosphorescent organ from above by transmitted light, showing the ramified network of 

 pigmented strings. 



Fig. 13. The pigmented network much more enlarged. 



Fig. 14. The same still more enlarged, showing its relation to the hexagonal pigmented cells. ;/, pigmented 

 strings ; r, hexagonal pigmented cells. 



Fig. 15. Ramified pigment-cell from the connective tissue layer beneath the phosphorescent organ. 



Fig. 16. Structiu-e of the connective tissue layer beneath the phosphorescent organ ; a, a fcAv hexagonal columns 

 left standing on the layer; h, connective tissue and other elements; c, nerve; '7, blood-vessel; 

 e, ramified pigment-cells. For purpose of clearness the other elements are omitted where the 

 pigment-cells are represented. 



Fig. 17. Special arrangement of pigment at the margin of the phosphorescent organ. 



