168 
PROF. D. M. S. WATSOK AND MR. E, L. GILL ON THE 
intertemporal and then to that of the squamosal. This groove is for a line 
of pit organs, found also in Ceratodus. Such grooves are independent of 
the bundles of strife already referred to. The corresponding bones of very 
young individuals show these grooves much more plainly (see fig 4, A, B, C, 
below). The course of the grooves, when they appear at all, is precisely that 
of the sensory grooves marked on the skull of Dipterus. 
One common type of skull-roof is not represented in fig. 2, p. 165, because 
it apparently always came to pieces in fossilization, and is hence only known 
by detached bones. It is characterized by the high polish of the external (as 
well as of the internal) surface of the bones, by their simplified outline, and 
by the conspicuous zonal bands near their margins. A fish of this type was 
evidently the predominant Sagenodus in Fritsch's collection from the 
Fig. 4. 
Sagenodus. Parietals, x about 3. A and B, of very young individuals, show well-maried 
lateral-line grooves and illustrate the scale-like form of the young bones ; C is of a 
rather older individual ; D is a bone of the zonal and polished type ; though small, it 
has the adult form. 
G-askohle; in the Atthey Collection it is comparatively scarce and usually 
of very small size, but a few large "parietals" prove that in the Northum- 
brian coalfield, as in the Bohemian, it reached the same size as the other 
forms. A very small " parietal " of this zoned and polished type is 
represented in fig. 4, D, above. 
The posterior margin of the skull-roof presents as a rule a concave out- 
line, sometimes pronouncedly so (fig. 2, E & F, p. 165 ; fig. 18, A, p. 183). 
In a few cases it is nearly straight or even slightly convex. A pair of 
tabular horns for the attachment of the shoulder-girdle are conspicuous at 
the back of a well-preserved skull. At the level of the inner face of these 
horns there is a thin frill of bone (fig. 1] p. 164, occ.f.) stretching across the 
