182 Mr. T. Stock on the Genus Tristychius. 
segmented, and, moreover, that the segmentation had proceeded 
so far as to be represented by distinct centra. So far as I am 
aware, there is no very convincing evidence at present avail- 
able that any Carboniferous shark possessed centra. Nume- 
rous biconcave vertebree have indeed been discovered, espe- 
cially in the English coal-measures ; but further observations 
are wanting before any of these can be referred with certainty 
to the skeleton of a Selachian fish. 
Thenext specimen to be describedis preserved in anodule, and 
is valuable as affording information more particularly with re- 
gard to the exoskeleton of the fish. The nodule measured about 
83 X7x3 inches before it was broken up for examination. 
I found it at Hailes Quarry, near Edinburgh, in the Caleci- 
ferous-Sandstone series. The part protruding from the bank 
of shale had become a good deal weathered, and a consider- 
able fragment must have at some time dropped off and become 
lost. The weathering has extended inwards for an inch or 
two, which is so far a fortunate circumstance, as a better view 
is got of the teeth than would have been possible otherwise. 
The rest of the nodule is hard and unweathered ; and though 
T succeeded in developing the spines and several more teeth, 
something has no doubt escaped detection *. 
Description.—The teeth are seen to be numerous; but it is 
impossible accurately to estimate their number, as they lie in 
much disorder, and many of them are very imperfectly seen. 
One of the largest (Pl. VII. fig. 9, nat. size) is 34 lines 
wide and 1} line high. The middle cusp is well developed, 
and is flanked by lateral cusps, three on each side (one of 
them is broken away in the example figured). These decrease 
in height, first suddenly as compared with the middle cusp, 
then more gradually with regard to each other. The cusps 
are strongly grooved. ‘The grooves are flexed, and diminish 
in width as they converge towards the apex. The wide spaces 
that separate the cusps are also deeply grooved. This deep 
sulcation gives the tooth a strikingly ornamental and character- 
istic appearance. The inserted portion is short, and covered 
with equally spaced narrow ridges (Pl. VII. fig. 10). These 
ridges are apparently prolonged beyond the base into root-like 
attachments, and in some specimens they appear to bound 
cavities in the bases of the teeth. It is also worthy of note that 
the free portion of the tooth is very oblique to the base, a nar- 
row groove marking the division between them. The other 
teeth differ from the one described chiefly in the varying depth 
of the sulcation and in the distances the cusps are set apart 
* Shark nodules are particularly refractory under development. 
