Cepuatopopa | UPPER PALA®OZOIC MOLLUSGA. 571 
OrTHOCERAS (Actinoceras) GIGANTEUM (Sow.) 
Ref. and Syn. = Orthoceras giganteum Sow. Min. Con. t. 246 = Actinoceras Simmsi Stokes, Geol. Trans, 
2nd Series, Vol. V. t. 59. fig. 4 and 5. 
Desc.—V ery large; section nearly circular; gradually tapering in most part of the length, but sometimes 
abruptly diminishing considerably in diameter in a short space ; septa moderately convex, distant, but irregularly 
unequal in distance and in the direction of their edges, which most usually however are even, and at right angles 
to the long axis of the shell; siphon large, slightly (less than its diameter) eccentric, greatly dilated into 
depressed, spheroidal beads between the septa, each swelling twice as wide as long, and longitudinally marked 
with close, small, coarse, irregular plice ; surface apparently smooth, or with very fine transverse striz of 
growth, Diameter of the dilated interseptal first siphon one inch when the shell is three and a half inches 
in diameter. 
A small regular specimen, imperfect at each end, containing part of the last chamber, five inches in 
diameter at the large end, and sixteen inches long, is one inch eleven lines in diameter at the small end, where, 
for five inches, the septa are slightly more than half an inch apart ; at four inches in diameter the septa are nine 
lines apart ; one very unusual fragment, about four inches in diameter at the large end, and about five inches 
long, is two inches three lines in diameter at the small end, the tapering being so rapid that the profile 
of the sides is concave; the edges of the septa are slightly oblique, but unequally so, the two chambers 
nearest the larger end are only six and four lines wide between their septal edges, while the septa towards the 
small end are regularly nine to ten lines apart : in another specimen, one inch ten lines in diameter, the septa are 
regularly nine lines apart; the siphon where it perforates the septum of the large end is four lines in diameter, 
and three lines in diameter at the small end. Some compressed specimens, eight inches in diameter, have seven or 
eight septa in succession, one inch and half apart. Some perfect terminal chambers in the collection having only 
two or three of the penultimate chambers adhering to them, shew the perfect last chamber alone 13 inches in length, 
and rather more than a foot in diameter, the lateral measurements being exaggerated by compression. Sections 
shew the siphon, in some specimens, to be rather more eccentric at the small than at the large end, and each of 
the dilated portions between the septa to be divided in the middle by a transverse partition; the space between 
the internal tube and the dilated wall being filled by vertical, fan-shaped, testaceous, radiating lamellz. One 
very young fragment, imperfect at each end, one inch five lines in diameter at the small end, and two 
inches four lines at the larger, tapers very regularly, and has a regular, broad-oval section, with the small 
diameter ;> of the larger, and has the septa four lines apart at the large end, and about half a line less 
at the smaller. 
This gigantic shell seems to have the section more nearly circular in old specimens than in young 
ones; and in the former also the siphon is more nearly central ; the size of the siphon varies very little with the 
diminution in size, so that the dilated portion occupies a much larger proportion of the small than of the 
large specimens. The septa are moderately convex, much less so generally than in Sowerby’s large figure. 
Position and Locality—Very common and of great size in the dark carboniferous limestone of Lowick, 
Northumberland ; very common in the red carboniferous limestone of Closeburn, Dumfriesshire, 
ORTHOCERAS INEQUISEPTUM (Phill.) 
Ref—Phill. Geol. York. Vol. II. t. 21. f. 7. 
Dese.—Very gradually tapering; section very broad-oval, nearly circular; a specimen one inch long 
is four lines in greatest diameter at the large end, and two lines in diameter at the small end; lesser diameter at 
the large end one fourth of a line less than the greater diameter ; septa very numerous, five occupying the space 
of one diameter in specimens varying from a little more than three to a little more than two lines in diameter 
(about four in a diameter when a little smaller), moderately concave, their edges nearly even, and at right angles 
to the long axis of the shell, but very slightly waved; surface not distinctly seen, but apparently smooth ; 
last chamber occupying half an inch of the above sized specimen ; siphon moderately large, nearly central. 
