272 LAKE SUPERIOR. 



f 



parietal bones. Small plates continue on the snout to its extremity, 

 and are prolonged on the sides before the nostrils, but do not reach 

 the lower circumference of the eye. All these bones are covered 

 with fine granules, disposed in linear rows in the direction of the 

 head. The eyes occupy the upper region of the face. They are 

 oval and have their largest diameter longitudinal. They are sur- 

 rounded with a smooth zone on their lower circumference, limited 

 above by the bones of the skull, and behind by a bone which sepa- 

 rates them from the opcrcular apparatus and the branchial cavity. 

 Another bone, which is triangular, being the continuation of the 

 preceding, limits the posterior margin of the face and completes the 

 anterior margin of the branchial cavity. The nostrils, situated in a 

 depression which is reserved for them before the eyes, open, as is 

 common, at the surface, by two holes pierced laterally, of which the 

 upper, the smallest, is subcircular and free, the lower oblong, vertical 

 and protected by a small membrane at its anterior margin. The small 

 plates which cover the snout reach not so far as the bone of the 

 lower angle of the face. The opercular bone is covered with these 

 fine granules disposed in striae radiating from the centre. The 

 membrane which invests it and which shuts the respiratory opening 

 in front, is covered with a fine rasp, which continues on the sides 

 of the head to the angle of the mouth. The branchiostegal mem- 

 brane proper is naked and very thin. It surrounds the opercular 

 bone from the upper margin of the branchial cavity, and is prolonged 

 and becomes wider a little above the branchial opening behind the 

 pectoral fins and beneath the head. 



The inferior surface of the head is level, with the snout a little 

 raised. The mouth opens in a depression behind the eyes. Its 

 general form is the same as in the A. Icevis, (see pi. 5, f. 2. ;) 

 it is protractile as in this latter, but the membranous fold which 

 surrounds the jaws, is smooth on its whole anterior circumference, 

 where it appears only as a wrinkle surrounding the jaw. It thickens 

 at the angles of the mouth and terminates in a flattened flap, of 

 glandular appearance, on the third quarter of the extent of the lower 

 jaw, leaving the symphysis bare. The palate and the tongue have 

 sinuous and transverse wrinkles on their surface. 



Four thread-like barbels half an inch long, are placed mid-way 



