40 PALEONTOLOGY OF NEW YORK. 



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

 Whitewater, Wisconsin ; D. Schtnidti, 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. 



I'LATK IV c, FIGS. 1-18 ; and PLATE IV D, FIGS. 1-15. 



1871. MoTwmerella, Billings. Canadian Naturalist, vol. \ i, p. 220. 



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



Meeting of British Association. 



1874. Monomerella, Davidsoi' and Kino. Quarterly Journal Geological Society, vol. xxx, p. 155. 



1875. Manomerella, Nicholson. Rept. PaliEontology of the Pi-ovince of Ontario, p. OS. 



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

 1884. Morunnertlla, Whiteaves. Palseozoic 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. Mu.scular impression on the platform very large, covering the 



