BIGSBT PALAEOZOIC EOCKS OF NEW YORK. 263 



The Welsh Dimyaria (64 species) show themselves 57 times in 

 calcareous deposits, mostly in the arenaceous limestone of Bala; 

 while we find them 69 times in arenaceous muds with and without 

 mica, and commonly of the Upper Ludlow period. We only see 

 the deep-sea or limestone beds thus fail comparatively in three 

 other cases in Wales — for instance, in the family Plantce and the 

 order ffeterojpoda and class Pisces. The Cleidophorus ^lanulatushoth. 

 of Wales and New York is in the same kind of argillo-arenaceous 

 matrix. The nine appearances of Pteropoda in New York are prin- 

 cipally in calcareous shales or argillaceous limestone, once in pure 

 limestone ; the eleven of Wales are pretty equally distributed over 

 deep and shallow sea-bottoms. The New York Gasteropoda reside 

 almost altogether in calcareous strata. Of the 94 species the 

 mineral positions of which are known, six only are found elsewhere. 

 It is different in Wales, — every sediment, except carbonaceous shale, 

 possessing one or more of these fossils. Out of 132 appearances, 75 

 are in lime-rocks, and principally in arenaceous (Bala) and argil- 

 laceous limestone. Considering that the Gasterojpoda are usually 

 littoral, it is remarkable that the arenaceous deposits of New York 

 should contain so few, because in that area the Silurian strata in 

 which a siliceous or argillaceous sand is the governing ingredient are 

 3000 feet thick, neither the argillaceous shales (800 feet thick) 

 nor limestones in their several varieties (1100 feet thick) attaining 

 much more than one-third of that thickness. The explanation is to 

 be found in the sandstones there being at the base of great or con- 

 siderable epochs. In New York, 95 out of 103 appearances of 

 the order Cephalopoda have calcareous habitats, — eight only, or 

 3^th of the whole, being met with in sandstone. In Wales, they are 

 more equally distributed : one-third of all the appearances (114 in 

 number) are in other than lime-rocks, seventeen of these being in 

 argillaceous sandstone, and eighteen in mudstone. The arenicolous 

 Orthocerata of New York are Mid-silurian, while in Wales two- 

 thirds are Upper Silurian. In both countries they are found in beds 

 containing mica disseminated in small scales. 



h. Argillaceous strata. — There may be in New York fossiliferous 

 strata of the Silurian age which are pure clay or mudstone ; but I 

 know of none. In Wales, 160 Testacea are found in mudstone, 

 and more especially the BracMopoda, Dimyaria, and Cephalopoda, 

 also the Crustacea. 



In abundance of fossils, argillaceous strata stand next to the cal- 

 careous ; and when lime and alumina meet together in certain pro- 

 portions, the Invertebrate population is extraordinarily great. 



The Graptolites of New York (16 in number) adhere closely to 

 strata abounding in clay, almost all of them being in the Hudson- 

 River group. Four are in argillaceous sandstone, and one in argil- 

 laceous limestone (Trenton group. Hall). The Graptolites of Wales 

 are most of them in mudstone and carbonaceous shale. Of the 

 51 appearances of Echinodermata in New York, thirty-five are in 

 shale, usually very aluminous ; but in Wales there are but six out 

 of 82 appearances so placed, — a discrepancy not difficult of explana- 



