1894.] XELEOSTEAST MOBPHOLOGY. 423 



Common Sole, my attention was arrested by a sac-like structure 

 lying immediately behind the lower eye, external to the membranous 

 wall of the orbital cavity. The absence of any mention of such a 

 structure in Cunningham's monograph of that species 2 suggested 

 that the specimen under examination might be in that respect 

 abnormal ; but I have since found it in every Sole which I have 

 examined, and there can be no doubt but that it is a regular 

 feature in the anatomy of the species. 



The organ in question is almost certainly homologous with a 

 rounded process of the membranous wall of the orbital cavity 

 discovered by Dr. Giinther 2 in Chorisochismus denteoo, one of 

 the Gobiesocidse, and conjectured by him to represent the saccus 

 lacrymalis of higher animals. Beyond this I have been able to 

 hnd no reference to the existence of any structure at all corre- 

 sponding to that now under consideration. 



It was my intention, before publishing, to work out the develop- 

 ment of the organ as completely as possible, since any opinion 

 that might be formed as to the homologies of the structure would 

 be of little value unless supported by a knowledge of its ontogeny. 

 The material at my command, however, has proved unsuitable for 

 the purpose, and, since some considerable time must elapse before 

 fresh specimens of the required stages are available, I have thought 

 it best to put forward such information as I have already collected, 

 imperfect as it is, in order to attract the attention of others who 

 may find themselves in a position more favourable for the pro- 

 secution of the inquiry. 



I have ventured to apply to the organ the name (Becessus orbi- 

 talis) which appears at the head of this section. 



Distribution in Species. 



I have found the organ present, in some form or another, in 

 every species of flat-fish examined for this purpose, and believe 

 that it will be found to occur in all. The species examined com- 

 prise the Halibut (Hijipoglossus vulgaris), Long Rough Dab (Hippo- 

 glossoid.es platessoides), Plaice (Pleuronectes platessa), Flounder (P. 

 flesus), Common Dab (P. limanda), Lemon " Sole " (P. micro- 

 cephalus), Common Sole (JSolea vulgaris), and Brill (Rhombus 

 Icevis). 



Topography and Structure. 



The organ consists usually of a sac-like process of the membranous 

 wall of the orbit, and in all the forms mentioned, except the 

 Halibut, both orbits are furnished with such a process. The 

 condition in the Plaice may be taken as fairly representative of 

 that met with in the genera Pleuronectes, Hippoglossoides, and 

 Solea, so far as their anatomy is known to me in this respect. 



1 'A Treatise on the Common Sole,' Plymouth, 1890. 

 - Cat, Fish Brit. Mus. vol. iii. p. 490. 



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