236 Transactions. — Zoology. 



face closely applied against the posterior surface of the skull. Each cornu 

 fits closely against the three facets already mentioned, and is strongly 

 attached by fibrous tissue to y and z, as well as to the outer border of x. 



Over against the auditory fontanelle the wall of the bladder becomes 

 considerably thickened, forming a pad {p) which fits tightly into the fora- 

 men and comes in close contact with the lamina of the fontanelle, its surface 

 presenting folds corresponding to those in the lamina. 



The outer or free end of each cornu of the air-bladder comes in close 

 contact with the thin skin (fig. 3, a) immediately in front of the dorsal end 

 of the shoulder-girdle and beneath the operculum (op). 



The arrangements described must form a fairly efficient transmitting 

 apparatus to the organ of hearing. Sonorous vibrations meeting the thin 

 subopercular skin, will be transmitted to the air in the air-bladder and 

 thence to the auditory fontanelle, the vibration of which will act imme- 

 diately on the perilymph. The subopercular skin will thus act as an im- 

 perfect tympanic membrane, the air-bladder as a tympanic cavity, and the 

 auditory fontanelle as a fenestra ovalis. 



EXPLANATION OF PLATE XXXIII. 



Fig. 1. Hinder view of the skull of Lotella bacchus (uat. size). 



au.f, right auditory fontanelle. op. o, opisthotic. 



b. o, basi-occipital. pt. o, pterotic. 



e. o, ex-occipital. s. o, supra-ocipital. 



ep. o, epiotic. x, y, z, facets for attachment of air- 



/. m, foramen magnum. bladder. 



I, lamina filling up left auditory fon- IX, X, foramen for exit of ninth and 



tanelle. tenth nerves. 



Fig. 2. Anterior end of air-bladder opened from the dorsal side ; p, pad fitting against 



auditory fontanelle ; r, rete mirabile. 

 Fig. 3. Transverse section taken just behind the head (diagrammatic). 

 a, place where air-bladder comes in ces, oesophagus, 



contact with subopercular integu- p, pad of air bladder. 



ment. sp. c, spinal cord. 



br. c, branchial cavity. v. 1, centrum of atlas vertebra. 



ly, lymphatic gland (so called head-kidney). 



