284 Proceedings of the Royal Physical Society. 
that in Agassiz’s list the name testudinarius occurs before 
cornutus, and must therefore be preferred, according to the 
usual rules of nomenclature. . 
The more I study the subject, the more I become con- 
vinced that still further reduction of the supposed species 
of Pterichthys is necessary. Accordingly, I not only heartily 
concur in the suppression of quadratus, and its fusion with 
Milleri, but I also would hand over testudinarius (including 
cornutus) to the same fate. The only difference between 
P. Millert and P. testudinarius which struck me in 1888 
was that, in the former, the under surface of the carapace 
was “broadly ovate,” while in the latter it was “narrowly ” 
so. But the subsequent re-examination of this question has 
convinced me that this is a character on which no reliance 
can be placed, and that therefore ¢estudinarius, like guadratus, 
must disappear from the list. 
There remain for consideration the two forms P. productus 
and P. oblongus,in which the terminal joint of the arm is 
peculiarly expanded and abruptly pointed. This character 
alone distinguishes P. productus from P. Muilleri, and as these 
two forms constantly occur together in the same beds, I 
must own that I am strongly possessed with the idea that 
the difference in point may be a sexual one. But in P. 
oblongus we have an additional character in the peculiar 
narrowness of the ventral aspect of the carapace, of which 
the edges are also nearly straight; and if this mark be 
specific, and the expansion of the terminal joint sexual, then 
the opposite sex of oblongus has yet to be discovered. It 
seems more probable that there is here no distinction of 
species, and that the difference in form of the under aspect 
of the carapace is merely varietal, though we can also readily, 
in any collection of specimens of Pterichthys, pick out those 
which show the characters of oblongus. 
In this condition I must therefore leave the question. 
Though strongly suspecting that there is but one species of 
Pterichthys in the Lower Old Red Sandstone of Scotland, and 
that the distinction of the arms is a sexual one, I cannot 
prove that this is the case. I therefore—judging from what 
seem to be actual residual differences, after the elimination 
