152 
StTOpJlCllosici llOVTBSCGUS^ Ide^n, 1866, ibid., p. 81 (non. de Verneuil). 
Fi'oductus scabriculus, J. Armstrong, 1871, Trans. Geol. Soc. Glasgow, III, suppL, 
p. 40. 
,, symnietricus, F. B. Meek, 1852, Eept. Pal. East. Nebraska, p. 167, pi, 5, fig. 6 
(pi. 8, fig. 13 esclusa). 
,, nebrascensis. Idem, 1872, iUd., p. 165, pi. 2, fig. 2. pi. 4, fig. 6, and pi. 5, fig. 11. 
This shell is usually a trifle longer than broad, the young individuals 
are generally sub-circular, and the adults nearly rectangular ; in the latter 
case the margins are slightly curved, and sub-parallel. The ventral valve is 
strongly ventricose, depressed in its median part, and usually furnished with 
a very broad, shallow, ventral furrow. The beak is pointed, although tumid 
at a short distance from its extremity, recurved and rather prominent. The 
hinge line is straight, and rarely as long as the transverse diameter of the 
shell ; the ears are small, and terminate in an acute angle. The whole 
external surface is covered with longitudinal ribs, remarkable for the alter- 
nate elevations and depressions they exhibit, transforming them into tubercles 
disposed rather irregularly in a quincunx, and frequently ornamented with 
small curved sj)ines. The dorsal valve is slightly concave, and usually fur- 
nished with a broad dorsal ridge corresponding to the ventral furrow, but the 
tubercles are here replaced by punctures of the same shape. The surface of 
both valves is generally crossed by. concentric undulations, better marked on 
the ears than elsewhere. 
Dimensions. — This species can acquire rather large dimensions, and it 
is not unusual to find them six or seven centimetres long. The only speci- 
men found by Mr. W. B. Clarke is twenty-three millimetres long by twenty- 
two broad. 
Delations and Differences.— Thi^ species has been figured under 
divers names, according to the greater or less perfect state of its preservation. 
After inspecting the original specimens, I am sure that most of the varieties 
named in the list of synonyms of this s23ecies depend merely upon the differ- 
ences in the kind of rocks in which they have been preserved, and on the 
greater or less ease with which they have been separated from it. 
Horizon and Localities. — This Droductus is not uncommon in the 
Upper and Middle Series of the Carboniferous Limestone. It occupies a hori- 
zontal position of great extent, being found abundantly in Europe, where the 
