122 ALUS. [Vol. XVIII. 



about 45°. The antero-inferior end of the facet passes onto the 

 cartilaginous interspace in front of the element, and almost 

 touches the postero-ventral corner of the first basibranchial. That 

 part of the depression portion of the bone that lies posterior to 

 this facet lodges that part of the first hypobranchial that lies im- 

 mediately posterior to its articular edge. The lateral surface of 

 the piece, posterior to the depression, is roughened, and gives in- 

 sertion to fibrous or semi-ligamentous tissues that bind the first 

 hypobranchial to the basal line, and at the same time connect the 

 first and second hypobranchials. 



The Third Basibranchial (BB^) is as long as the first two 

 together, and about one half as deep. It is connected with the 

 second basibranchial by synchondrosis, and ends posteriorly in 

 cartilage. Its dorsal surface is rounded and smooth. The ante- 

 rior quarter of its lateral surface is deeply cut away by the articu- 

 lar facet for the hypobranchial of the second arch, this facet re- 

 sembling in every respect the articular facet on the second basi- 

 branchial, but being somewhat smaller than that facet and some- 

 what less strongly inclined downward. Posterior to the facet 

 there is, as on the second basibranchial, a roughened surface, but 

 it lies much more on the ventral than on the lateral aspect of the 

 bone. It gives insertion to fibrous or semi-ligamentous tissues 

 that bind the second hypobranchial to the basal line, and at the 

 same time connect that hypobranchial with the hypobranchial of 

 the third arch. The dorsal surface of the bone, between the 

 dorso-posterior ends of the facets of opposite sides of the head, 

 is narrow, and the ventral surface, between the ventro-anterior 

 ends of the facets, is reduced to a narrow line. The antero-infe- 

 rior end of the facet, like that of the facet on the second basi- 

 branchial, extends onto the cartilaginous interspace in front of the 

 element, but not onto the bony element in front of that interspace. 

 Both of these facets thus belong to a basal element numerically, 

 in the nomenclature used, one posterior to the branchial arch to 

 which they give articulation. 



Posterior to the roughened surface that lies immediately behind 

 the articular facet for the second hypobranchial, the third basi- 

 branchial is deeply grooved on its ventral surface through about 

 one half the remainder of its lensfth. The anterior end of this 



