426 MB. E. W. L. HOLT — STTDIES IN [May 1 , 



eating with the cavity of the orbit to which it belongs through a 

 foramen in the skull. In all but Rhombus, the organ lies partly 

 in front of the superficial jaw-muscles, immediately under the 

 skin, and partly between those muscles and the skull. In figure 8 

 the organ of the Halibut is displayed by the removal of the anterior 

 part of the muscles, and by clearing away such part of a great 

 pad of adipose matter as interfered with the view. It is seen that 

 the organ is divisible into an anterior and a posterior limb, the 

 division being due to the effort of the organ to accommodate itself 

 to the available room — viz., in front of and internal to the muscles 

 named. An arrow shows the passage of the anterior limb to 

 a funicular region leading to the foramen through which access 

 is gained to the general orbital cavity. The foramen in question 

 is that between the parasphenoid and the bony bridge formed 

 by the union of the ectethmoid and sphenotic of the blind side : it 

 aiso gives exit to a palatonasal branch of the Y-cranial. 



The organ of the blind side in its internal structure is similar 

 to that of the ocular side in such forms as Pleuronectes and Solea. 

 It is somewhat noticeable that it is rather larger in the Halibut 

 than in any of the species which possess a definite sac-like organ 

 of the ocular side, but the description given for the Halibut is 

 sufficiently applicable to all the forms enumerated except the 

 Brill. In this species 1he difference is brought about by the more 

 forward extension of the jaw-muscles, which completely overlie 

 the organ. In consequence it is flattened : it is semicircular in 

 outline, the arc being ventral, and communicates with the orbital 

 cavity by a short narrow neck arising from the centre of its dorsal 

 surface. It is smaller than that of any other species mentioned, 

 but agrees with them in internal structure, and is considerably 

 larger than its fellow of the opposite side. 



Figure 7 of Plate XXX. shows a dissection which exposes the 

 upper, or left, orbital cavity in the Plaice, a dextral flat-fish, by the 

 removal of a great part of the right and left frontals, including the 

 whole of the bony interorbital septum, and of the right and part of 

 the left ectethmoid. It is seen that the orbital cavities are now sepa- 

 rated only by the fibrous band which forms the internal (morpho- 

 logically ventral) continuation of the interorbital septum. The 

 recti muscles and the optic nerve have been cut through, ami the 

 membranous wall of the cavity has been slit along the margins of the 

 eye to allow of the latter being turned forwards. The orbital cavity 

 is thus fully exposed ; it is seen to be roughly ovoidal in shape, w ith 

 a conspicuous funnel-like depression on the left side near the 

 posterior end. The membranous wall, which lines the whole of 

 the cavity, is continued into this funnel, which is the opening 

 into the accessory organ which we have already studied from 

 the other side. A seeker can readily be passed through the 

 opening, and the organ can easily be injected with fluid through 

 the medium of the orbital cavity. It is worthy of notice that 

 the upper orbital cavity is bounded on most sides by structures 

 which yield little or not at all to pressure. In front and behind 



