LOWER HELDERBERG ROCKS. 
and stronger lines of growth, which are numerous in the older shells. 
Internally the ventral valve is marked by a large foliate vascular 
impression : impressions of the adductor muscles rarely well pre¬ 
served, except in the casts. Teeth prominent, and, when entire, rounded 
and thickened at their extremities. Dorsal valve Avith a prominent 
cardinal process and divergent brachial lamellce : a central ridge, more 
or less prominent, extends from beneath the beak to near the base. 
This species resembles Orthis hybrida of the Niagara group, but is much larger, 
Avith valves proportionally less convex, and never so straight in front : the area is 
likewise smaller and shorter. In the young state it is more elongated, the beak of 
the ventral valve more extended, and the surface more coarsely striated than in 
corresponding specimens of 0. hybrida. The older shells become more circular and 
gradually less convex. The ventral valve is marked by a broad undefined depression 
down the centre, making the entire valve broadly concave from a little below the 
beak, and producing a sinuous outline in front. The dorsal valve maintains a 
generally uniform convexity, its greatest height being towards the beak. 
In young and half-grown shells the length and height are nearly equal, Avhile in 
older specimens the proportions of length and breadth are often as three to four. 
In old shells the strial become less conspicuous and comparatively finer than in the 
young shells; and this appearance is sometimes exaggerated by the process of 
silicification, which has affected the greater number of the Helderberg specimens. 
On comparing this with a similar species in the Hamilton group, we find the 
young more circular, and the beaks less prominent in that one than in the Ilekler- 
berg species. The surface is likewise more finely striated; and the ventral valve, 
though sometimes concave, has never any mesial depression, and the margin is not 
sinuate. There are likewise other differences, which may be observed on careful 
comparison of specimens. 
*b Individuals showing a gradation of size, from the smallest recognized specimens 
of the species to the half-grown forms. 
h ig. 4, 5, 0. Individuals illustrating a more elongate and more gibbous form, which seems 
scarcely separable from the others, but is still never found of larger size than 
fig. 6. 
big. 7, 8, 9, 10. Individuals of larger dimensions ; the last being a full-grown specimen of 
the broad variety, one and three-fourths inches wide by one and a quarter long. 
big. 11, 12, 13, 14. Ihe interior of the ventral valves of several specimens, showing some 
variety in the vascular impressions. 
Fig. 15, 16. Ihe interior of the dorsal valves of the more elongate and gibbous forms. 
