Dr. R. H. Traquair—On Tristychius and Ptychacanthus. 27 
ambulacral ossicles and the two adjoining adambulacral plates, and 
not through the body of the ambulacral ossicle itself. Hall has 
claimed to have discovered traces of a disc in Billings’s type 
specimens of Teeniaster, and if so, this distinction falls to the ground ; 
but, in regard to the two other points, the collection of specimens of P. 
Miltoni in the British Museum Collection confirm the correctness of 
Salter’s views. The differences are, therefore, valid ones, and both 
genera must be retained, as has been done by Zittel. This new species 
differs from Teniaster in the presence of the disk, though, as we have 
seen, this distinction may possibly not hold good, and in the structure 
of both arms and oral pentagon. It undoubtedly differs from the 
other species of Protaster in many important points of structure, 
and herein agrees more closely with Brisinga than does any Prot- 
ophiuroid yet described; to mark this resemblance the specific 
name has been given. But the other species of Protaster differ 
among themselves in equally important points, and the genus is 
apparently constituted by a group of species which careful revision 
and further knowledge of their structure would probably relegate to 
more than two distinct genera. The necessity for such a revision 
has been recently urged by Sturtz,! and to this with a discussion of 
some questions in their anatomy and the significance of some points 
in the structure of P. brisingoides, I hope subsequently to return. 
Till such has been done, it seems advisable to include this species in 
the comprehensive genus Protaster. 
VI.—Nore on THE Genera TRisTYcHius AND PTYCHACANTHUS, 
AGASSIZ. 
By Dr. R. H. Traquatr, F.R.S., F.G.S. 
Se years ago,?in showing that the spines supposed by Hancock 
and Atthey to be dorsal spines of Gyracanthns were merely 
young and unworn specimens of lateral spines of the same genus, 
T called attention to the general fact that young examples of Selachian 
spines could not be expected to represent the older ones in miniature, 
and vice versa. 
For as the spine increases in size by growth at the base, the young 
one is consequently represented only by the distal portion of the 
adult. And as in the process of growth, differences in sculpture and 
proportions may supervene, the general characters may be so altered, 
that if the distal portion be lost by attrition, the old and young 
individuals may be with difficulty recognizable even as belonging 
to the same genus. 
Such an instance may, I think, be found in the spines named by 
Agassiz respectively Tristychius arcuatus and Ptychacanthus sublevis, 
both from the Lower Carboniferous rocks of Central Scotland. | 
In very young specimens of the former the surface of the exserted 
portion is entirely covered with longitudinal ridges and sulci, and 
1B, Sturtz, Beitrag zur Kenntniss paleozoischer Seesterne, Palaeontographica, 
vol. xxxil. Stuttgart, 1886, p. 79. 
2 Ann. and Mag. Nat. Hist. ser. 5, vol. xiii. 1884, p. 37. 
