Mr. T. Stock on the Genus Tristychius. 179 
Tristychius fimbriatus, Stock, may be looked upon as distinct 
from, though probably related at no great distance to, Styra- 
codus acutus, Giebel. 
Tristychius arcuatus, Ag. 
This spine is rather common in the vicinity of Edinburgh. 
I have possessed or seen in other collections nearly a hundred 
examples. It appears to be commonest on the Burgh Lee 
horizon (Edge Coal), probably from the fact that the beds 
there have been pretty carefully examined ; but it also extends 
down to some of the lowest beds of the Calciterous-Sandstone 
series of the district. I am acquainted with it from Abden, 
near Kinghorn, Fife (Hdge Coal ?), South Queensferry (Bur- 
diehouse), Grange Quarry, Burntisland, Fife (Burdiehouse) ; 
Juniper Green (Wardie, collected by Mr. Henderson, and 
presented by him to the Museum of Science and Art) ; Hailes 
Quarry (Wardie); and from Carolina Park (Granton). A 
consideration of this ample material has elicited a few facts 
worthy of being put on record. 
I have figured (Pl. VII. figs. 7 & 7a) the specimen col- 
lected by Mr. Henderson, and now in our collection, partly as 
being a good typical example of the larger form of the species, 
and also to show that the denticles near the point merge into 
a ridge with a ragged (scarcely denticulated) edge. The 
general shape has been preserved; but the proximal half has 
been much injured. On the side, at the point, there is 
a very small smooth area; and immediately below it the 
ridges and furrows are pretty strongly marked. Three 
distinct but not very prominent parallel ridges extend back 
from the anterior margin for a short distance; and the rest of 
the area is in this specimen finely and interruptedly striated. 
There is, however, a great range of variation in various 
specimens as to the strength of the sulcation and ridging. 
Some are nearly smooth, and those the largest; others, 
usually smaller specimens, are often strongiy ridged and sul- 
eated. ‘These differences are so apparent in different examples 
that for a long time I thought a stable specific distinction 
might be made out between them; but wider experience has 
convinced me that a clear gradation can be easily traced be- 
_ tween them. 
Having now discussed the fragmentary evidence upon 
which the genus 77istychiws has been hitherto chiefly known, 
we are able profitably to advance to the consideration of certain 
specimens in which that evidence is combined with other 
facts of a much higher order of importance. Amongst these 
the first to call for notice is a considerable fragment of the 
13* 
