168 
PALEONTOLOGY OF NEW YORK. 
stones, the shells show the fact that the squamae of earlier growth, or those 
upon the umbonal and median surfaces of the valves, have been worn off during 
the life, or before the fossilization of the shell; the later squamae, which are 
stronger, broader, and more closely crowded about the margins, are those usually 
retained, and these are sometimes of great width, not infrequently equaling 
and sometimes exceeding the diameter of the valves."^ 
This form first appears in the Clinton group of the State of New York, and 
in rocks of corresponding age elsewhere in the United States. It continues its 
existence through the Niagara group, the Lower and Upper Helderberg groups, 
the Hamilton and Chemung groups, and into the fauna of the Lower Carbonif¬ 
erous, carrying the same features through all these periods, and presenting no 
variety of form or surface-markings which can be considered as more than 
variations of expression depending upon the surrounding physical conditions or 
similar influences. Nevertheless, in most of these successive faunas this type- 
form has, for each one, an expression so distinct and peculiar that these varia¬ 
tions, without accessory evidence, are often sufficient for the determination of 
geological horizons. 
The shells occurring in the Clinton group of New York and Ontario are 
characterized by their suborbicular form and the generally small size of the 
adult, f 
In the fauna of the Niagara group this form is continued, though its habit 
of growth is larger, and the concentric lamellm of the surface more closely set, 
as it prevails in the Niagara shales of New York. In the soft shales and 
limestone at Waldron, Indiana, it presents itself with greater rotundity or con¬ 
vexity of valves; at Louisville, Kentucky, a common form is a small, elongate 
rather than orbicular, shell, with characteristically obsolescent plication. Shells 
of the same character as the last also occur sparingly at Waldron and in New 
York. In the Lower Helderberg fauna the elongate variant prevails in the 
Shaly limestone, attaining a greater size than in the preceding fauna; while in 
* See Davidson, Silui'ian Brachiopoda, pi. xiv, fig’s. 1, 2. 
Bakrande, Sj’steme Silurien, vol. v, pi. xix, fig. 7. 
Whiteaves, Contrib. to Canadian PalsEontology, vol. i, pi. xxxvii, fig. 8. 
t It is hardly necessary, were it possible, to determine with precision which of the many expressions of 
Atrypa reticularis was borne by the specimens which served the Swedish savant as the type of the species. 
