38© 
PALAEONTOLOGY OF NEW-YORK. 
This species presents some variety of form, more particularly in the relative 
proportions of the two extremities; some specimens being almost symmetrical, while 
others are much narrower on the anterior side. The individuals likewise vary in the 
degree of convexity; and while in well-marked specimens the middle lobe is not 
above the lateral ones, there are others where this lobe rises in a rounded node above 
the plane of the lateral lobes. 
Fig. 3 a. The right valve of a symmetrical form of this species, enlarged four diameters. 
Fig. 3 b. Profile view of the same. 
Fig. 3 c. Figures showing some variety of form, as presented in a group of individuals upon 
the surface of a fragment of limestone : some of them are much more ventricose 
than others. 
Geological •position and locality. In the tentaculite limestone, and base of the 
shaly limestone of the Lower Helderberg group : Herkimer county. 
This species is often extremely abundant, covering almost the entire surfaces of 
some layers of the rock. 
The degree of variation in the forms which appear to be traceable to this species 
induces me to place the following as a variety of the same. 
ISeyricSda notata, var. ventiicosa. 
Plate LXXIX B. Fig. 4 a, b, c. 
Carapace unequally semielliptieal, ventricose, the anterior extremity 
much narrower than the posterior : two short furrows mark the sur¬ 
face ; the anterior one straight, and shorter than the other; the po¬ 
sterior one longer and slightly curving forward below, with a slight 
depression around the base of the middle lobe connecting the two 
grooves. Middle lobe consisting of a small oval tubercle, which does not 
reach the hinge margin. Posterior lobe narrow above, spreading and 
swelling out below, becoming quite ventricose and rounded on the ven¬ 
tral margin. Anterior lobe smaller and less ventricose, and separated 
on the ventral side by a slight sinuosity from the posterior lobe. Ven¬ 
tral margin incurved beneath the overspreading lobes, and appearing 
at the two extremities as a flattened rim, which, on the anterior side, 
at the junction with the hinge-line, spreads out in a subalate extension, 
and likewise in a less degree at the postero-cardinal extremity. 
Surface finely granulose. 
