282 PfiOCEEDINGS OF THE GEOLOGICAL SOCIETr. 



23 species — 16 being Lower Silurian, and 9 in the Trenton Lime- 

 stone alone. Wales has 11 species, and of similar position and rela- 

 tions. This genus arises in the Caradoc or Bala rocks in Wales, and 

 is of equal force in all the three stages. 



4. Birdseye Limestone. — Life exhibits very few forms here. The 

 most interesting is the American genus Bucania, a Gasteropod allied 

 to Bellerophon. It is present in all the three Silurian stages, in 12 

 species ; but it is far most numerous in the lowest. The same may 

 be said of the genus Modiolopsis, in 24 species (Dimyarian). The 

 bivalve Crustacean, Cytherina (?) as formerly called, is first seen 

 here. It has 5 species, each in a separate group, 3 being Upper 

 Silurian. 



5. Trenton Limestone. — The Annelida of New York commence their 

 existence here, excepting one, Scolithus linearis. They will before 

 long be proved to be more abundant than has been hitherto 

 suspected. Although few in this epoch, the species are known in 8 

 group-sections, which sections are in contact only in three instances. 

 This order prevails more largely in Wales, where we find it in all 

 the rock-groups which range from the Lower Bala to the Ludlow 

 inclusive. The genus Sj)infer Sjcst shows itself in the Trenton sec- 

 tion by two malformed species, just as in Wales (Upper Bala). It 

 is spread thinly over the Silurian system generally, and has two 

 places of greatest development, the Niagara and Delthyris Shaly 

 limestones. In the Devonian strata it multiplies suddeiily into 53 

 species, — 11 being in Corniferous Limestone, 19 in Hamilton, and 14 

 in Chemung rocks. The species oiSpirifer found principallyin Devon- 

 shire are, according to Phillips, 27 in number (' Manual,' p. 146). 

 Avicida, a Monomyarian genus, arises in the Trenton Limestone, and 

 spreads over 10 sections, in 22 species, principally in Upper Silurian. 

 There are 31 species in Middle Devonian of ISTew York, with 14 in 

 the Hamilton Rocks. Life is powerful in this genus ; for from the 

 Trenton Limestone it runs up to Chemung at the base of Old Red 

 Sandstone. In this section also we meet mth Tellinomya, a genus 

 of short duration in Silurian strata. Nine Dimyaria and Oastero- 

 'poda come into view here ; but they do not call for any observation. 

 The first Pteropod, Comdaria, is seen in the Trenton Limestone. The 

 Canadian, and perhaj)s the New York, Oonularia is in form and de- 

 coration more perfect than any English specimen which I have met 

 with. This genus only appears in the Silurian strata of New York 

 twice, and once in the Hamilton group of Mid-devonian. It has 

 only one form (G. Sowerhyi) in the Silurian of Wales. 



6. Utica Slate. — Although this section is of great thickness and 

 extends over very large surfaces, it originates few fossils. Thei'e 

 seems to be here an arrest of life, which is the more remarkable 

 from its being chiefly composed of the same argillaceous shale which 

 is so plentifal in the highly fossiliferous rock preceding it. 



7. Hudson-River Rocks. — In these strata fossil life multiplies con- 

 siderably, especially as to the Dimyaria (18) and Graptolites (7, 

 new). This might be expected from the nature of the matrix which 

 this section afibrds. 



