Royal Physical Society. 385 



with the inferior maxilla, and connected by ligament with 

 the intermaxillary bone ; and lastly, the pterygoid, which ar- 

 ticulates with the pre-frontal and the palate bone. The in- 

 ferior maxilla consists of only two pieces at each side, the 

 articular and the dental ; the last is armed with velvety teeth. 



The operculum consists of two pieces, massive in structure, 

 — the opercular bone, articulated by a complete joint to the 

 epi-tympanic bone ; and the inter-opercular bone, of a triangu- 

 lar shape, with its base attached by ligament to the anterior 

 margin of the opercular bone, and its apex to the articular 

 bone of the lower jaw. 



The infra-orbital bones are six in number on each side. 

 They are hollow tubes, in which mucus glands take their rise. 

 Five of them form a chain from the process of the post-frontal 

 bone to the side of the nasal bone, and the sixth passes from 

 the posterior extremity of this chain to the anterior process of 

 the mastoid. 



The supra-temporal bones, as already stated, are repre- 

 sented by a cartilage arising from the mastoid bone by two 

 origins, which unite so as to form a Y-shaped structure, which 

 terminates in the integument at the upper extremity of the 

 branchial aperture. 



A similar cartilage, of a linear shape, arises a little farther 

 back from the upper part of the coracoid bone, and also passes 

 outwards to support the integument of the branchial aperture — 

 this being one of the Siluridse which have the branchial aper- 

 ture narrow, and the skin hanging so loosely round it that the 

 branchiostegal rays are invisible beneath it. Probably this last 

 cartilage represents the clavicle, that bone being otherwise 

 absent. 



The Iiyoid arch consists of four pairs of bones. The su- 

 perior bone (the stylohyal) is reduced to a mere nodule like 

 the smallest pin-head. The remaining three are beautifully 

 united. The ceratohyal is a hollow cylinder, open at both 

 ends, and the epihyal and basihyal are hollow caps, which are 

 united to it by suture so as to form one bone, consisting of a 

 shaft and two epiphyses. The basihyal bone is separated from 

 its fellow by the apex of the urohyal, — a triangular bone with 

 the apex in front and the base behind, lying in the horizontal 



