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ALUS. [Vol. XVIII. 



of the shank there is sometimes a deep rounded groove which 

 looks like a partly enclosed foramen. It is lined, in the natural 

 condition, with a thin mucous membrane and marks the upper 

 limit of the gill opening between the two arches. The shank of 

 the bone is grooved on its dorsal surface, the groove turning 

 mesially around the anterior edge of the postero-mesial proc- 

 ess of the element and running off the postero-mesial edge of the 

 base of the large articular head. 



The Fourth Epibranchial {EB, /[') is a short bone bent 

 at right angles between its shank and its flat, irregularly triangular, 

 proximal end. The shank of the bone is but slightly or not at 

 all grooved on its dorsal surface, but it has, along its postero- 

 mesial edge, a rough, irregular ridge. Both ends of the piece 

 are capped with cartilage, the proximal cap being usually sepa- 

 rated into two portions, a large anterior one and a small posterior 

 one. The anterior cap articulates with the fourth infrapharyngo- 

 branchial, lying transversely to an articular strip of cartilage on 

 the dorsal surface of that element. The posterior cap lies on a 

 more or less distinct process of the element, and articulates, with- 

 out the intermediation of a cartilaginous facet, directly with the 

 dorsal surface of the fourth infrapharyngobranchial immediately 

 posterior to the cartilage that gives articulation to the main articu- 

 lar head of the piece. This posterior process of the epibranchial, 

 and the fourth infrapharyngobranchial, are strongly but loosely 

 connected with each other by ligament, the attachment of the 

 ligament to the infrapharyngobranchial being on a part of that 

 bone that is of cartilaginous origin and not on the adjoining 

 dermal part. ^ 



At the angle between the head and the shank of the epibranchial, 

 on its antero-lateral edge, there is a small process directed an- 

 teriorly and capped with cartilage. It articulates with the postero- 

 mesial process of the third epibranchial. The proximal head of 

 the element is, in its natural position, directed downward, back- 

 ward and mesially. 



The First Infrapharyngobranchial (IPB, I) is a small, 

 conical, curved bone with a large cartilaginous cap at its larger, 

 distal end, and a small one at its pointed, proximal end. It is 

 directed upward, laterally, and slightly backward along the ventral 



