40 PALjEONTOLOGY of new YORK. 



(Trenton) ; D. parvus, Whitfield, from the Galena limestone (Trenton) at 

 Whitewater, Wisconsin ; D. Schmidti, Davidson and King, is stated to be from 

 the Lyckholmer Schichten of Esthonia, Russia, considered to be nearly the 

 equivalent of the Caradoc and Trenton ; the greatest individual development 

 of the genus, however, is in the Niagara fauna and its equivalents, D. Conradi 

 being the American representative of D. Davidsoni, Salter, a Wenlock species 

 occurring near Dudley and elsewhere in England, county Kerry, Ireland, and 

 on the Island of Gotland ; D. transverstis and D. Woodwardi, Salter, are from 

 the same horizon, and the Trematis Bohemica, Barrande {= Dinobolus), is from 

 an equivalent fauna (Etage E-e.,). 



Genus MONOMERELLA, Billings. 1871. 



PLATE IV C, FIGS. 1-18; AND PLATE IV D, FIGS. 1-15. 



1871. Monomerella, Billings. Caiiaiiian Naturalist, vol. vi, p. 220. 



1872. Monomerella, Davidson and King. Ann. and Mag. Nat. Hist. ; Geol. Mag-. ; Report Biighton 



Meeting of Bi-itish Association. 



1874. Monomerella, Davidson and King. Quarterly Journal Geological Society, vol. xxx, \i. 155. 



1875. Monomerella, Nicholson. Rept. Palaeontology of the Province of Ontai'io, p. 68. 



1875. Monomerella, Hall and Whitfield. Geol. Surv. Ohio ; Palteont., vol. ii, p. 131, pi. vii, tigs. 1, 2. 

 1884. Monomerella, Whiteaves. Palaeozoic Fossils, vol. iii, pi. 1, pp. 5, 6. 



Diagnosis. Shell usually thick, but in some cases quite thin ; outline vary- 

 ing from elongate-ovate to subcircular. Surfaces of contact of the valves some- 

 times conspicuously broad. 



Pedicle-valve with a more or less elevated umbo, which may vary in height in 

 a given species, in the type-species being high, as in Trimerella; umbonal cavity 

 divided into two chambers by a longitudinal septum. Cardinal area large ; 

 deltidial ridges and deltidial callosities not always distinctly developed ; cardi- 

 nal slope usually well defined, merging anteriorly into the cardinal buttress, or 

 umbonal septum ; in some species this slope, or subtriangular area, is longi- 

 tudinally divided by a furrow. The crescent is sharply impressed over the 

 cardinal slope ; terminal scars broad and distinct. Platform more or less 

 developed ; divided by the cardinal buttress, which extends nearly or quite its 

 entire length. Muscular impression on the platform very large, covering the 



