160 A:\ifc;KiOAN jolknai. 



Genus TRIMERELLA, Bill. 



Trimerella, Bill., June, 1862, Pal. Fos. Geol. Sur, of Canada, 

 vol. i, p. 166. 



Shell ovate, moderately convex, solid, and transversely stri- 

 ated. The inner surface of each valve furnished with a longitu- 

 dinal septum, supporting a flat, horizontal, lozenge-shaped plate, 

 attached by its posterior edges to the inner surface of the valves 

 upon which the muscular apparatus was attached. On each side 

 small ovate muscular scars, as in Oholus. Htemal hinge line with 

 faint longitudinal ridges. 



Type. Trimerolla grandis, Bill., Pal. Foss. Geol. Surv. Can. 

 p. 166, fig. 151 a, b, 1862. 



Fossil in the Silurian beds of Canada. 



GOTLANDIA, Norn. geu. prov. 



Trimerella, Lindstrom, Ofv. K. Vetenskaps Ak. For. 1867, 

 p. 253, pi. xxi. Not Trimerella, Billings. * 



Shell subelongate, rounded before ; neural valve with an acute, 

 produced apex ; hcemal shorter and more rounded, provided with 

 a projecting, semi-oval, cardinal process, which is articulated 

 into a half-moon-shaped pit just before the area in the neural 

 valve ; hinge line of the latter very long and deeply grooved, 

 the two grooves meeting at an acute angle on the posterior end 

 of the valve. The hinge line of the htemal valve is simple, fit- 

 ting into the grooves just described, forming with the cardinal 

 process an articulation of the two valves. Neural septum rounded 

 at its anterior end, extending nearlj'- to the margin ; on each 

 side of the septum and closely attached to it is a tubular process 

 extending as far back as the hinge margin, where the base forms 

 the concave anterior wall of the cardinal pit. The top of the 

 septum is lower than the concave surfaces of the tubes and forms 

 the bottom of a gutter between them. The anterior ends of the 

 tubes are nearly in the middle of the valves. Similar tubes exist 

 in the hsemal valve, and the perforations are shown by a section 

 to extend nearly to the apices. They are represented in casts 

 by conical slender processes. The area behind the cardinal pit 

 is covered with callus in the form of arched lamellge. The mus- 

 cular impressions have not been clearly made out, but appear to 

 have been situated on the upper, posterior ends of the tubes. 



Loc. — Silurian beds of the island of Gotland. 



This genus, as will be seen by the diagnoses, is quite distinct 

 from Trimerella, Billings, though the latter has faint ridges on 



