OCR Text |
Show 442 MR. E. W. L. HOLT-STUDIES I N [May 1, of the head the disposition of the muscles was found to be almost normal, the only difference being that the ventral face of the anterior dorsal muscles, winch normally exhibits an almost semicircular indentation for the reception of the dorsal hemisphere of of the upper orbit, is, in the specimen before us, only slightly concave in the same region. The space usually occupied by the upper eye was filled with a mass of connective tissue, and to some extent also by an extension of a pad of gelatinous and adipose matter, which is always found above the right nasal organ. Bemoving this, the right ectethmoid and the interorbital septum have the usual appearance from this aspect, and the right orbitonasal nerve is seen passing as usual below the anterior notch of the left ectethmoid. There is nothing in the condition of the right orbital apparatus that calls for remark. The bony ridge on the left edge of the top of the skull is, as usual, united to the ventral face of the dorsal muscles, and to the great sickle-shaped ligament-bone 1 imbedded therein, by a very tough white ligament. The morphology of the Pleuronectid skull having been very clearly defined by Traquair2 many years ago, and that of the Common Sole having received special attention in M r . Cunningham's monograph on that species, it is unnecessary for m e to refer except very briefly to the normal features of its component parts. In any Flat-fish the top of the skull in the orbital region consists of two bony ridges. Of these, that on the side of the fish occupied by the eyes consists of the interorbital septum formed by the coalesced anterior limbs of the two frontals (very unequally developed of course) running forward to the ectethmoid of the ocular side. The opposite ridge, that of the blind side, is the pseudomesial process of Traquair, formed by the union of an anterior process of the sphenotic of the blind side with a posterior process of the ectethmoid of the blind side, and between these two ridges is the upper orbital cavity, the optic nerve and oblique and recti muscles thus reaching the eye. Accordingly in the normal Sole w e find the nerve and muscles of the left or upper eye to the right of the pseudomesial process. In the abnormal Sole, as I have already mentioned, the eye is not on the right side of this process, and in fact is completely shut off from the right side of the head by the fibrous connection of the process to the ventral face of the dorsal muscles. In fact, all that is to be seen of the left orbital apparatus consists of the 1 This bone, which supplies a firm base of attachment for the dorsal muscles to the skull, is not found in Bhombus and Pleuronectes, the genera to which the observations of Cunningham and M a c M u n n limit the occurrence of the Cyclopean malformation. It is possible that its presence in the Sole may supply the wished-for structural peculiarity correlated to the non-occurrence of the malformation in Solea, since, as the authors remark, the dorsal fin in Solea actually extends further forward than in the Turbot, and much further than in the Flounder, both of which are commonly Cyclopean (see these authors, 02?. cit. p. 806). It m a y be remarked, however, that the burrowing-habits of the Sole might well preclude the survival of specimens so malformed, since the longish dorsal hook would be a serious inconvenience. 2 Trans. Linn. Soc. Lond. xxv. p. 263. |