STRUCTURE OF CERTAIN PALEOZOIC DIPNOI. 
179 
symphysis, sometimes considerably more so than in fig. 15, C, p. 180, and 
together they must have produced a small backward process at the middle 
line (fig. 16, B, p. 181), a difference that would be reflected in the shape of 
the gular plates which adjoined the " dentaries " behind. 
Fig. 13. 
'sp! 'den.' 
'den! 
ang. 
ang. 
'spr- 
ang. 
ang. 
E F 
Ceratodus {Neoceratodus) fontteri. The bones and tooth-plates of the lower jaw. A, dorsal, 
B, ventral, 0, lateral, D, mesial aspects; E, "^spleuial," from the outer side; 
F, ano-nlar, from the mesial side. (Reference letters as in fig. 12.) 
The Gular Plates. 
The absence of gular plates has been given as one of the diagnostic points 
separating the Ctenodoiitidse from the Dipteridse. The similarity of the' 
hinder border of the " dentaries " in Sagenodus and Dipterus led us, however, 
to look for something in Sagenodus to correspond with the gLilars which are 
applied to this border in Dipterus, and this we believe we have found in tlio 
bone represented in fig. 16, A, p. 181. There are three examples of tliis 
bone in the Atthey Collection. One of them occurs on a slab of shale 
