1^6 ALUS. [Vol. XVIII. 



of the indentation forms a point, more or less sharp, near the mid- 

 dle of the hind edge of the bone. Ventral to this point the hind 

 edge of the bone is concave and overlaps externally, through its 

 entire length, the anterior edge of the suboperculum, the two 

 bones being firmly bound together by tissue. The antero-ventral 

 edge of the operculum is overlapped externally by, and bound to, 

 the hind edge of the preoperculum. 



The Suboperculum (SOP) lies with its greatest length 

 directly nearly vertically, and forms a large part of the posterior 

 edge of the gill cover. The dorsal portion of its anterior edge 

 is overlapped externally by the hind edge of the ventral part of the 

 operculum ; the ventral portion of the edge being overlapped by the 

 hind edge of the interoperculum. Between these two overlapped 

 portions, the bone has a strong process directed dorsally and for- 

 ward and ending in a sharp point. The process lies immediately 

 antero-ventral to the antero-ventral edge of the operculum, and 

 internal to the preoperculum, to which latter bone it is firmly bound 

 by fibrous tissues. 



The Interoperculum {I OP) is large and somewhat oval in 

 shape. Its long axis lies in a nearly horizontal position. Its ven- 

 tral edge is nearly straight, runs forward and slightly upward, 

 and forms the entire ventral edge of the gill cover. Its hind end 

 is rounded, and overlaps -externally the suboperculum, to which 

 it is firmly bound. Its dorsal edge is also rounded and is consider- 

 ably overlapped externally by the ventral edge of the preoper- 

 culum, which edge is sometimes nearly parallel with the ventral 

 edge of the interoperculum. The anterior end of the bone is 

 bluntly pointed and is bound by ligament to the hind end of the 

 mandible. 



At the middle point, approximately, of the dorsal edge of the 

 interoperculum, the bone is firmly bound to the ceratohyal at or 

 near the point where the latter bone gives articulation to the 

 epihyal. The ventral edge of the bone extends ventrally beyond 

 the ventral edge of the ceratohyal, and is there bent inward in 

 an abrupt curve. The two or three most dorsal branchiostegal 

 rays lie, when at rest, enclosed in this curved surface, the inter- 

 operculum having every appearance of being simply an enlarged 

 ceratohyal, or epihyal, ray. Whether it is the homologue of the 



