LOWER HELDERBERG ROCKS. 163 



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 with a prominent 

 cardinal process and divergent brachial lamellae : 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, 

 with 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, while in 

 older specimens the proportions of length and breadth are often as three to four. 

 In old shells the striae 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 si)ecies in the Hamilton group, we find the 

 young more circular, and the beaks less prominent in that one than in the Helder- 

 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. 



Fig. 1, 2, 3. Individuals showing a gradation of size, from the smallest recognized specimens 



of the species to the half^grown forms. 

 Pig, 4, 5, 6. 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. 

 Fig. 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. 

 Fig. 11, 12, 13, 14. The interior of the ventral valves of several specimens, showing some 



variety in the vascular impressions. 

 Pig. 15, 16. The interior of the dorsal valves of the more elongate and gibbous forms. 



