168 PALEONTOLOGY OF NEW YORK. 



stones, the shells show the fact that the squamae of earlier growth, or those 

 upon the urabonal and median surfaces of the valves, have been worn oft' 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 surfiice-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, t 



In the fauna of the Niagara group this form is continued, though its habit 

 of growth is larger, and the concentric lamellse 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, Silurian Brachiojioda, pi. xiv, tigs. 1, 2. 

 Barrande, Systeme Silui-ien, vol. v, pi. xix, fig. 7. 



Whiteaves, Contrib. to Canadian Palaeontology, vol. i, pi. xxxvii, fig. 8. 

 f It is hardly necessary, were it possible, to determine with precision which of the many expressions of 

 Atnjpa re.ticidaris was borne by the specimens which served the Swedish savant as the type of the species. 



