AN EURYPTERID HORIZON IN THE NIAGARA. 
5 
DESCRIPTION OF SPECIES. 
Lickenalia concentrica Hall. 
Plate I, figure 1. 
Specimens fragmentary; surface generally undulatory ; 
radial and concentric striae about 0 • 5 mm. apart. In both sets of 
striae there is a tendency for some of the furrows to be much 
stronger than others. 
From the above described characters, there appears to be 
little doubt that this is L. concentrica , described by Hall from 
the Rochester shale and lower Lockport limestone at Rochester 
and Lockport, New York. 
Monomorella cf. orbicularis Billings. 
Plate I, figure 2. 
One specimen, which I have compared with the above species, 
occurs in the collection. It is 26 mm. wide and 27 mm. long, 
the shape being suborbicular with the greatest width forward 
of the centre of the shell. The specimen is an exfoliated pedicle 
valve. The growth lines are well preserved, but the interior 
structure is not visible except at the beak, where wear has 
exposed the median septum and the fillings of the ends of the 
umbonal chambers. I have compared this specimen with Bill- 
ings type (internal mould) of M, orbicularis and find much 
similarity. However, the preservation of the two specimens 
is so entirely different that it is very difficult to be certain of 
their identity. I have noted some differences in the proportions 
of the umbonal cavities as represented in the two specimens 
and I have consequently compared the present specimen with 
M. orbicularis instead of making a definite determination. 
Orbiculoidea subplana (Hall). 
Plate I, figures 3, 4, 5. 
This species, which is represented by many individuals, 
is suborbicular with axes averaging about 5 and 8 mm. Valves 
