matter for histological study. The remains of approxi- 

 mately 20 of the formerly separate dental units can be 

 seen along the edge of the premaxillary to either side of 

 the midline and to a depth of 5 to 10 units posteriorly 

 from the edge. At the posterior end of the tooth bearing 

 region, the sockets surrounding the individual pri- 

 mordia open to the exterior on the outer surface of the 

 premaxillaries by small pores, similar to those described 

 for Triodon. A large trituration plate is present in the 

 dorsomedial region of the inner surface of the fused pre- 

 maxillaries. The trituration plate is divided into right 

 and left halves whose medial edges are in close contact. 

 Each plate has its relatively flat surface marked by ap- 

 proximately five parallel grooves which show the limits of 

 the originally separate dental plates that are now fused 

 together in a solid mass. These dental plates are formed 

 in a large pulp cavity, above the trituration plate, which 

 is separated from the pulp cavity in which the teeth of 

 the jaws are formed by a thin bony partition and which is 

 broadly open to the exterior posteriorly to either side of 

 the midline. The newly developed platelike trituration 

 teeth are obliquely placed in an anterodorsal to postero- 

 ventral plane at about a 45° angle to the surface of the 

 trituration plate. At first, only the posteroventral edge of 

 the new tooth plate is exposed at the surface of the 

 trituration plate, but as the tooth is moved forward and 

 downward, its whole substance is gradually worn away. 

 As the tooth plates are moved forward they become more 

 and more closely packed together, until they lose most of 

 their individual identity, at least in larger specimens. 



Maxillary. — Expanded ventrally where it forms the 

 lower one-third of the anterior margin of the upper jaw; 

 broadly overlies and extensively interdigitates with the 

 premaxillary. The medial surface of its free ventral end 

 articulates by fibrous tissue with the dorsolateral surface 

 of the dentary. Posterodorsally the maxillary possesses a 

 medially directed process which runs under the anterior 

 end of the palatine to make fibrous tissue contact with 

 the lateral edge of the posteromedial projection of the 

 fused premaxillaries. The concavity on the posterodor- 

 sal edge of the maxillary articulates by fibrous tissue 

 with the anterior end of the palatine. In about the middle 

 of its length the maxillary is laterally expanded into a 

 heavy flange which serves as a place for muscle at- 

 tachment. 



Lower Jaw. 



Dentary. —The two dentaries are indistin- 

 guishably fused in the midline, and, like the premaxil- 

 laries, form a huge crushing beak in conjunction with the 

 fused teeth. The concave posteromedial surface of the 

 dentary interdigitates with the lateral surface of the ar- 

 ticular. Posteroventrally the dentary interdigitates with 

 the angular. A large trituration plate, divided into right 

 and left halves in close contact with one another, is pre- 

 sent in the medial region of the inner surface of the fused 

 dentaries. The structure and development of the dental 

 units of the trituration plate and biting edge of the lower 



jaw are exactly like that described for the premaxillary, 

 except that the internal cavity for the dental pulp is 

 somewhat smaller. The dorsolateral surface of the den- 

 tary articulates by fibrous tissue with the ventromedial 

 surface of the maxillary. 



Articular. — More or less triangular in shape; its 

 posterior edge laterally expanded basally and bearing a 

 groove for fibrous tissue articulation with the quadrate; 

 cartilage filled for only a short distance anteriorly where 

 it is continuous with the remains of Meckel's cartilage; 

 articulates by extensive interdigitation anteriorly with 

 the dentary and posteroventrally with the angular. The 

 sesamoid articular is a nubbin of bone, of variable size, 

 which is usually interdigitated with the articular just 

 above and posterior to the remains of Meckel's cartilage. 



Angular. —A small bone interdigitated anteriorly 

 with the dentary and dorsally with the eu'ticular. 

 Posteriorly the angular attaches by ligament to the inter- 

 operculum. 



BRANCHIAL APPARATUS. 



Hyoid Arch and Branchiostegal Rays. 



Hypohyals. — Dorsal and ventral hypohyals 

 present; the ventral hypohyal about twice as large as the 

 dorsal hypohyal; dorsal hypohyal cartilage filled at its 

 ventral edge, ventral hypohyal cartilage filled at its pos- 

 terior edge; articulate through cartilage with one an- 

 other and with the anterior end of the ceratohyal; ar- 

 ticulate by fibrous tissue medially with their opposite 

 members. 



Ceratohyal. —Somewhat expanded posteriorly; 

 cartilage filled at its anterior and posterior edges; ar- 

 ticulates through cartilage anteriorly with the hypo- 

 hyals, and through cartilage and slight interdigitation 

 posteriorly with the epihyal, which it overlies. All six 

 branchiostegal rays articulate by fibrous tissue with the 

 ceratohyal, the platelike first ray with a vertical groove 

 on the medial surface in about the middle of its length, as 

 explained below. 



Epihyal. — Round and broadly overlain by the pos- 

 terior end of the ceratohyal, with which it articulates 

 through cartilage and slight interdigitation. The lateral 

 surface of the epihyal articulates by fibrous tissue with 

 the medial surface of the ventral flange of the inter- 

 operculum. 



Branchiostegal rays. — Six in number; the first 

 branchiostegal ray a large plate, of increasing width pos- 

 teriorly, whose lateral edge is sharply downturned and 

 whose medial edge is more gradually upturned. Anterior- 

 ly the downturned lateral edge of the first branchio- 

 stegal becomes thickened into a vertical articular face 

 which fits against the vertical groove on the medial sur- 

 face of the ceratohyal. The second branchiostegal ray is a 



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