ON VARIATIONS IN FORM IN SALMO FONTINALIS. 



13 



aspect, the myelon the " neural " aspect of his body, as in the 

 animals below him whether vertebrate or invertebrate. 



Profile diagram of head and brain of insect, with fore part of the neural and 

 haamal tracts or centres, in the position thereby indicated. 



The letters of reference are : — a. Hasmoesopliageal centre or " ganglion " = 

 fore brain, b. Neurcesophageal centre or " ganglion "=hind brain, c. (Eso- 

 phagus traversing the crura cerebri, or connecting-cords, d, to the neurostome 

 in its course, e. Nerve (olfactory ?) to antenna. /. Optic nerve, g. Ocellar 

 nerve, h. Mandibular nerve, i. Lingual nerve, k. Maxillary and labial 

 palpal nerves. I. Stomach, or alimentary axis. m. Heart, or h^mal axis. 

 n. Ganglionic cords, or neural axis = myelon. o. Foremost thoracic centre or 

 lion." 



On Yariations in Form and Hybridism in Salmo fontinalis. 

 By Mr. Francis Day, F.L.S. 



[Read November 2, 1882.] 



There are few investigations more interesting in ichthyology 

 than ascertaining the amount of variation which a given species 

 of fish is capable of undergoing while adapting itself to new 

 conditions of life ; and there does not appear to be any form 

 more susceptible of change, when introduced into new regions, 

 than members of the genus Salmo. As opportunities occur of 

 observing any modifications, I think it highly desirable that such 

 should be recorded ; for even if unimportant when taken alone, 

 they may prove a link in some future inquiry. 



During the past twenty years many additions to our knowledge 

 of the natural history of members of this genus have resulted 



