116 CARBONIFEROUS LAMELLTBRANCHIATA. 



Genus Syncyclonema, Meek, 1864. 



Syncyclonema, Meek, 1864. Smithsonian Check-list N. Amer. Cret. Foss., p. 31. 

 Entolium, Meek, 1864. Geol. California, vol. i, p. 478. 



Generic Characters. — Shell Pectiniform, ovate, compressed, equivalve; ears 

 small, triangular. In the left valve the anterior larger, and both projecting' 

 upwards; umbones small and acute, depressed and central. The triangularly 

 shaped body of the shell marked off by two broad diverging grooves from the 

 expanded anterior and posterior margins. No byssal orifices beneath the 

 anterior ear. 



Interior. — Hinge with a small cartilage pit at the centre, and a linear, hori- 

 zontal groove on either side. Two deep, oblique, strong ridges diverge from the 

 umbo, triangular in shape, terminating abruptly, seen as grooves in casts, probably 

 hinge-teeth. Starting below these teeth, and internal to them, broad, oblique, 

 shallow ridges pass downwards and towards the margins, separating the body of 

 the valve from the curiously expanded borders. The adductor muscle-scar is 

 large, shallow, central, and close below the umbonal region. 



Exterior. — The surface is almost smooth, or covered with concentric strias and 

 lines of growth. In decorticated specimens fine radiating strias and zigzag 

 markings are seen, especially near the lower border of the valve. 



Observations. — When describing the peculiar characters of P. Sowerbyi in 1844, 

 M'Coy stated that it agreed with certain Oolitic forms, but mentioned no species. 

 Later on (' Brit. Pal. Foss.,' 1855, p. 478) he referred his species to the genus 

 Amusium, Megerle, from which it seems to differ solely in the absence of internal 

 radiating ribs, and possibly in the gaping lateral margins. To Amusium M'Coy 

 referred Pecten Sowerbyi, M'Coy, and P. deomatus, Phillips, but I cannot accept the 

 latter shell as belonging to the same genus as the former. P. deomatus has not the 

 peculiar shape of 8. Sowerbi/i, and Phillips describes it as follows : — " This has 

 scarcely distinguishable characters, yet contrasts with the others by its smooth 

 concentric furrows." Had this type, which, indeed, has unfortunately disappeared, 

 possessed the peculiar characteristics of Syncyclonema, Phillips conld not have 

 described it in these terms. 



In 1864 Meek proposed the name Entolium for a Jurassic species which has all 

 the important characters distinguishing the Carboniferous shell. He reviews this 

 new genus at length (' Min. Rep. Nebraska' [U. S. Geol. Surv., 1872], p. 190) 

 when describing a shell from the Carboniferous beds of Nebraska, which is 

 probably identical with the British species. He writes : — " At the time of proposing 

 this name I was under the impression that the valves of these shells were closed on 



