one another. The teeth are borne in very shallow depres- 

 sions on the surface of the premaxillary in their fully 

 formed condition. They develop, however, from large 

 deep sockets which open to the surface just anterior to 

 the base of the old tooth which they will replace. As the 

 tooth erupts to the surface to replace an old tooth, the 

 socket from which it arose seems to become overgrown or 

 filled in with bone and to disappear so that the tooth 

 rests flatly against the surface of the premaxillary. Most 

 of the interior of the premaxillary is given over to the 

 dental pulp cavity, this cavity communicating with the 

 exterior through a foramen in the posterodorsal surface of 

 the bone as well as through numerous smaller holes on 

 the inner and outer surfaces of the bone. 



Maxillary. — Widest ventrally, becoming narrower 

 dorsally; articulates firmly with the premaxillary by fi- 

 brous tissue and slight interdigitation. The medial sur- 

 face of the ventral region of the maxillary articulates by 

 fibrous tissue with the lateral surface of the upper por- 

 tion of the dentary. The posterodorsal surface of the 

 maxillary articulates by fibrous tissue with the lateral 

 expansions of the ethmoid and vomer and with the 

 medial surface of the palatine. 



Lower Jaw. 



Dentary. —Wide posteriorly; its posterior edge con- 

 cave to accommodate the anterior portion of the ar- 

 ticular, with which it articulates by interdigitation. 

 Posteroventrally the dentary articulates by inter- 

 digitation with the angular. From the lateral surface of 

 its posterodorsal region the dentary articulates by fi- 

 brous tissue with the medial surface of the maxillary. 

 Ventromedially the flat surface of the dentary articulates 

 by fibrous tissue with its opposite member. Each den- 

 tary bears four teeth in a single row, corresponding to the 

 outer row of teeth in the premaxillary. The teeth are 

 borne flush against the surface and are replaced by new 

 teeth developing in deep sockets, just as described for the 

 teeth of the upper jaw. Most of the interior of the dentary 

 is filled with the dental pulp, the cavity opening to the 

 exterior at the concave posterior edge of the dentary, as 

 well as at pores on the inner and outer surface of the 

 bone. 



Articular. — Wide along its posterior edge, with a 

 concave facet for articulation by fibrous tissue with the 

 anterior knob of the quadrate; articulates by inter- 

 digitation with the concave posteromedial surface of the 

 dentary. Along its ventral edge the articular inter- 

 digitates with the angular. The sesamoid articular is a 

 small but thick nodule of bone closely held by fibrous tis- 

 sue to the medial surface of the articular just in front of 

 the concave articular facet of that bone. 



Angular. — Small, squarish; articulates by inter- 

 digitation dorsally with the articular and anteriorly with 

 the dentary. Posteriorly the angular connects by liga- 

 ment with the interoperculum. 



BRANCHIAL APPARATUS. 



Hyoid Arch, Branchiostegal Rays, and Urohyal. 



Hypohyals. —Both hypohyal elements well de- 

 veloped, the ventral element considerably larger than 

 the dorsal element; the dorsal element cartilage filled at 

 its dorsal Eind posterior edges; the two elements ar- 

 ticulate with one another through the cartilage that lies 

 between them; the dorsal and ventral elements ar- 

 ticulate by fibrous tissue at their anteromedial edges 

 with their opposite members. The dorsal hypohyal ar- 

 ticulates by fibrous tissue at its dorsal edge with the 

 lateral surface of the first basibranchial. The ventral 

 hypohyal articulates through cartilage at its posterior 

 edge with the ceratohyal, and by fibrous tissue from its 

 ventromedial surface with the urohyal. 



CeratohyaL — A short shaft, wider posteriorly than 

 anteriorly; cartilage filled at its anterior and posterior 

 edges; articulates through cartilage anteriorly with the 

 ventral hypohyal; articulates posteriorly through car- 

 tilage and interdigitation with the epihyal. The first two 

 branchiostegal rays articulate with slight depressions on 

 the ventral edge of the ceratohyal. The ventralmost one 

 or two rays of the remaining four rays have their fibrous 

 tissue articulation principally with the ceratohyal. 



EpihyaL — Large; cartilage filled at its ventral 

 edge; articulates through cartilage and interdigitation 

 ventrally with the ceratohyal, while dorsally it ar- 

 ticulates by fibrous tissue with the interhyal and inter- 

 operculum. 



InterhyaL- A short, thick rod; cartilage filled at 

 both ends; articulates by fibrous tissue ventrally with the 

 epihyal and dorsally with the symplectic. 



Branchiostegal rays. — Six in number; increasing 

 slightly in length posteriorly in the series; first ray con- 

 siderably flatter and wider than the others. The first two 

 rays articulate by fibrous tissue with shallow grooves on 

 the ventral edge of the posterior half of the ceratohyal. 

 The other four rays articulate by fibrous tissue with the 

 ceratohyal and epihyal. The more ventral of these four 

 rays have their connective tissue fibers attached prin- 

 cipally to the ceratohyal, while the more dorsal rays have 

 them attached principally to the epihyal. 



Urohyal. — Thick along its dorsal and anterior 

 edges, otherwise a thin plate; articulates by fibrous tis- 

 sue anteroventrally with the medial surface of the ven- 

 tral hypohyal, while posteriorly it articulates with the 

 ventral surface of the first two basibranchial elements. 



Branchial Arches. — All the elements are cartilage 

 filled at their edges of articulation with the other 

 elements of the series, and the articulations are usually 



105 



