Prof. H. A. Nicholson — On the ' Guelph ' Limestones. 347 



strom, Hall, Davidson, and King have made palaeontologists so 

 thoroughly acquainted with. The most abundant members of this 

 family are Trimerella grandis, Billings, T. acuminata, Billings, Mono- 

 merella prisca, Billings, and Dinobolus Galtensis, Billings. In Ohio, 

 the most abundant species is Trimerella Ohioensis, Meek, which, 

 though very local in its distribution, is in places common enough. 

 After the Trimerellids, the most characteristic and abundant form is 

 the great Pentamerus occidentalis, Hall, which is found both in 

 Canada and the United States. In Ohio, however, the formation is 

 so richly charged with Pentamerus oblongus, Sow., that it is often 

 termed on this account the " Pentamerus Limestone ;" whereas this 

 well-known shell is in Canada, curiously enough, almost confined to 

 a thin bed at the base of the Niagara Limestone. So strictly is this 

 the case, that the bed in question, under the name of " the Penta- 

 merus band," has usually been employed by the Canadian geologists 

 to separate the Niagara Formation from the underlying Clinton 

 Formation. Another not uncommon form is Pentamerus (Penta- 

 merella) ventricosus, Hall ; whilst other forms have been recorded 

 belonging to Spirifera, Charionella, Strophomena, etc. 



The Lamellibranchiata of the Guelph formation, so far as Canada 

 is concerned, appear to be wholly referable to the remarkable genus 

 Megalomus. Casts of the interior of the large and massive Megalomus 

 Canadensis, Hall, are found in almost all the localities where the 

 Guelph Limestones have been detected, though nowhere so abun- 

 dantly as in Western Ontario. In some places, as in the cliffs of the 

 Grand Biver below Flora, whole beds appear to be made up of 

 this bivalve; but it is difficult to obtain specimens in which the 

 actual shell is preserved. Another smaller species of the genus was 

 described by Mr. George J. Hinde and myself from the Guelph for- 

 mation of Hespeler, under the name of Megalomus compressus (Cana- 

 dian Journal, vol. xiv. p. 143, fig. 6). In the Guelph Limestones of 

 Wisconsin, in addition to Megalomus Canadensis, Prof. Hall has 

 described species of Lamellibranchiata belonging to the genera Am- 

 bonychia, Pterinea, Avicula, Cypricardinia, Modiolopsis, Amphicoelia, 

 Cypricardites, and Palceocardia. 



Perhaps the most abundant and characteristic fossils in the Guelph 

 formation — at any rate in Canada — are, however, the Gasteropoda. 

 The three genera which are most largely represented are Murchisonia, 

 Pleurotomaria, and Holopea, and of the first-named of these the 

 variety of species is something quite extraordinary. Speaking 

 generally, the following may be cited as being the most abundant 

 and characteristic forms of the Gasteropoda in the Guelph formation : 

 — Murchisonia macrospira, Hall ; M. Logani, Hall ; M. turritiformis, 

 Hall; M. Vitellia, Billings; M. bivittata, Hall; M. longispira, Hall; 

 M. Estella, Billings ; M. Hercyna, Billings ; M. Laphami, Hall ; 

 Cyclonema (?) elevata, Hall ; C. sulcata, Hall ; Pleurotomaria solari- 

 oides, Hall ; P. Elora, Billings ; P. Galtensis. Billings ; Eolopea 

 Sarmonia, Billings ; H. Gracia, Billings ; H. Guelphensis, Billings ; 

 Trochonema fatua, Hall ; Subulites ventricosa, Hall ; Bucania angus- 

 tata, Hall ; Strapacollus Daphne, Billings ; and S. Mopsus, Hall. 



