BIGSBY PALEOZOIC JROCKS OP NEW TOEK. 



273 



Wales, as grouped in orders. This little Table tells us that Plantce, 

 Annelida, Crinoidea, and Pisces are the most tenacious of their 

 original sea-bed, and that Brachiopoda, Heteropoda, ZoopTiyta, and 

 Monomyaria are the most discursive. AVe furthermore infer from 

 this Table that the typical fossils of Wales are three times the number 

 of the recurrent (as 604 to 195), and that the typical of each order 

 are always in excess. Among the Crinoids there is only one recur- 

 rent — the Periechocrinus moniliformis* . 



The next Table (XIY.) gives the proportions of typical to recur- 

 rent species as they exist in the different stages of the Silurian rocks 

 of New York and Wales. We see from this Table, that the fossils 

 of the lower and middle Silurian stages of New York display nearly 

 the same amount of vertical range, but the upper stage sensibly 

 less, — and this partly from the proximity of a new and dissimilar 

 condition of beds in what is called the Devonian system. In Wales, 

 the vertical range is greater than in New York, and is most remark- 

 able in the middle stage. 



g. Recurrence of Orders and Genera. — Although but few species 

 enjoy repeated recurrence — only 67 in Wales, and 28 in New York 

 (see Table XI.), — most of the orders and genera (the Zoophyta and 

 Bryozoa, for instance) occupy very many successive groups of strata. 

 By an inspection of the general Tables of the fossils of New York 

 and Wales, the student may so easily verify these facts for himself, 

 that we shall not put the results here in a tabular form. We also 

 notice many instances in which the orders and genera of these two 

 distant countries are similar in the number, great or small, of the 

 epochs they occupy : and many more examples of this perfect or 

 proximate agreement would be found, were their respective fossils 



Table XIY. — Silurian Species in Wales and New York. 



Silurian 



Stages. 



i 

 New York. 



Wales and Siluria. 



o 



g 



o 

 9^ 



No. of First 



Appearances 



of Recurrents. 



Proportion of 

 Recurrents 

 to Typical. 





1 



No. of First 



Appearances 



of Recurrents. 



Proportion of 

 Recurrents 

 to Typical. 



Upper 277 



Middle ' 127 



Lower 386 



45 

 25 

 71 



20 

 28 

 65 



•166 

 •21 

 •24 



316 



60 



243 



113 



117 



63 



40 

 55 

 63 



•29 



•257 



•32 



more in common ; but New York abounds in special or regional forms. 

 Table XV. exhibits in a striking manner the great similarity 



* The student can easily draw up a similar Table for the Silurian Fossils of 

 New York, from Table I,, Quart. Journ. Geol. Soc. toI. xiv. p. 399. 



