174 RAMSAY H. TRAQUAIR ON 
From the appearance presented by one specially large mandible, I rather 
suspect there is a third small one, as there is in Rhizodus, just below the 
symphyseal extremity of the dentary, and I have in my paper in the “ Annals” 
referred to some doubtful evidence of still another, situated posteriorly on the 
lower margin of the jaw, and here separating the angular from the first infra- 
dentary for a little distance, but on this I am not prepared to insist. 
We have as yet accounted for the attachment of one laniary tooth, the one 
at the symphysis. But the mandible of Rhizodopsis, when perfect, shows not 
merely one large tooth in front, but several additional ones (usually three in 
number) behind it and internal to the series of smaller teeth. What has 
become of these in the dentary bone when disarticulated and detached ? 
A ready explanation of this is found in the structure of the lower jaw of 
certain Old Red Sandstone “Dendrodonts” in which the laniary teeth are not 
attached to the dentary bone proper,-but to a series of accessory “internal 
dentary ” pieces articulated to its inner side.* Should this also be the case 
with the posterior laniaries of the mandible of Ahzzodopsis, then in cases where 
its elements are broken up and separated, these additional pieces will also get 
detached, and the absence of all but the anterior laniary in the isolated dentary 
bone will thus be amply accounted for. 
At the time I wrote the notice in the ‘‘ Annals,” already quoted, I had not 
obtained a clear view of the ossicles supporting the posterior laniaries in 
Rhizodopsis, and consequently referred to the analogy of the structure of the 
lower jaw in Ahizodus, in which I had most certainly found them, as amounting 
to a moral certainty of their existence also in the former genus. My attention 
has subsequently been directed to a specimen in the Edinburgh Museum of 
Science and Art, which completely confirms the view I then took. 
This is a slab of shale, not localitated, but probably from the Edinburgh 
Coal Field, over which scales of Rhizodopsis of large size lie thickly scattered, 
some of which are over 1 inch in length and nearly ? in breadth. ‘This is 
indeed an unusually large size, but is by no means an isolated example of the 
bulk which Rhizodopsis must sometimes have attained, and the form and 
sculpture of the scales here exhibited unmistakeably demonstrate the genus 
to which they belong. Lying in the midst of the scales is a mandible, 
evidently belonging to the same fish, and seen from the internal aspect. 
The splenial is gone, as is likewise the bony substance of the symphyseal part 
of the entire mandible, though a rough impression of it remains on the stone ; 
the hinder extremity is also injured, as well as the posterior part of the lower 
margin ; such impressions of the external surface, as remain when the bone has 
splintered off, indicate a sculpture of the usual minutely pitted-rugose character 
* See Panper’s “ Saurodipterinen, Dendrodonten, &c., des devonischen Systems,” pp. 41-43, 
tab. x. figs. 2, 3, 4, 14, 22, 
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