336 SIR R. OWEN ON A LABYRINTHODONT AMPHIBIAN. 
more directly on this aspect; the latter are remarkable for their 
small relative size, as this is indicated by the fore and part of the 
upper boundary; their aspect is obliquely lateral. Each of the 
above-described bones has a sculptured outer surface of the same 
pattern. Froma central or subcentral space, radiate narrow, almost 
sharp ridges with wider intervals; similar but shorter ridges rise 
as the intervals expand toward the periphery of the bone; the 
ridges become irregularly wavy as they diverge thereto: the inter- 
vals are smooth grooves. One or two wider channels, suggestive of 
haying carried mucous canals, indent some of the bones. This 
character suggested the generic name Rhytidosteus. A very small 
portion of the maxillary bone, 21, comes into view in the upper 
surface of the skull; it is the part (21, fig. 1, Plate XVI.) which 
contributes the hind border of the nostril. The firm adhesion .of 
the lower to the upper jaw forbade further attempts at severance ; 
the hardness of the cementing matrix threatened such to be more 
. destructive than expository of determinative characters. In the 
interval of the diverging rami of the portion of mandible so pre- 
served (Plate XVII. fig. 1) sufficient of the bony palate was exposed 
to afford the chief batrachian character of the present labyrin- 
thodont genus. The major part of this mouth-roof is contributed 
by the pair of vomerine bones, 13, of which the mesial suture is 
conspicuous. Kach vomer presents a large subcircular vacuity, 
vc, 20 millim. in longest diameter, from which projected the 
crown of a laniary tooth, the exposed basal part not exceeding 
6 millim. in diameter*. The disproportion between this tooth and 
the socket or vacuity from which it was emerging suggested that it 
might be a successor to a canine conforming in size with such 
socket, and which had been lost. Behind each of the vacuities, v ¢, 
is the palatal nostril n',n’, 28 millim. in length, 8 millim.in breadth ; 
the long axis of the aperture is parallel with the outer border of the 
bony palate. The anterior and mesial with part of the lateral or 
outer borders are contributed by the vomer ; the rest of the circum- 
ference by the palatal plate of the maxillary; and, perhaps, the 
palatine, 20. Close behind the palatal nostril is the socket or aper- 
ture of emergence of a canine, mo, similar and but little inferior in 
size to the vomerine one, vc. ‘To this palato-maxillary vacuity and 
tooth applies the same remark as to the vomerine one. 
The vomerine bones diverge from each other after a sutural 
course of two inches, and the pointed anterior end of a palatal 
bone undivided by any apparent trace of mid suture occupies the 
interspace. The breadth of the bony palate at the fractured hind 
border is 5 inches; the length of the part preserved along the 
mesial line is 45 inches. 
The mandibular rami, 31, 32, have coalesced at a symphysis, 
2 inches in length, 3 inches in breadth at the hind border, thence 
narrowing forward to a breadth of 1 inch 9 lines (45 millim.); and 
again slightly expanding to a rounded termination with a feeble mid 
indentation. The portion of each ramus cemented to the upper jaw 
* From this the section, fig. 2, Plate XVIL., was taken. 
