APPENDIX A. 
septa and form of which it imitates. The sinus of the mouth 
occurs on the higher side of the oblique septa. 
Loe.—Brigsteer, Westmoreland, in Ludlow rocks. 
ORTHOCERAS DIMIDIATUM, Sow. 
Syn.— 0. dimidiatum, Sow. in Sil. Syst. t. 8. fig. 18. Munster, 
Beitr. 1840, t. 19. fig. 2. 
Sp. Ch.— Slender, surface transversely undulated ; waves 
reaching but half across ; septa moderately distant.” Sow. 
The section appears to be oval, the ridges of growth (they 
are hardly undulations) are sometimes on the dorsal and some- 
times on the ventral side of the shell, and, as they are oblique, 
in one case they point backwards,—in the other, rise forwards 
from the margin. 
There are specimens of both in Mr Sharpe’s cabinet from 
Radnorshire. 
Loe.—Brigsteer ; and Derby Arms, near Kendal, in Ludlow 
rock. 
O. TorQuATUM, Munst. 
Syn.— 0. torquatus, Munst. Beitr. 1840, p. 101. 
Sp. Ch.—Cylindric, linear ; crossed obliquely by close strong 
strie, which are distant, oblique, and lost on the ventral side in 
the young shell, but less oblique and continuous across as it 
grows older; near the mouth the shell is crossed only by fine 
strie, some more prominent than the rest. 
Munster gives no specific characters to his species, and I 
have tried to supply one from tolerable specimens sent by him 
to Cambridge. The siphon is not visible, but he says it is cen- 
tral, and the septa close. 
I believe I cannot be wrong in referring a fragment found in 
the Ludlow rock of Kendal to this strongly-marked species; 
the specimen is of larger diameter than Munster’s before the 
change of striation takes place. 
Loc.—Helme Foot, Kendal, in Ludlow rock. 
O. privayvum, Forbes Sp. 
Syn.— Creseis primeva, Forbes, Geol. Journ. Vol. I. p. 146. 
Sharpe, ib. Vol. II. Pi.138. fig. 2. 
Sp. Ch.—Very long, linear, dilated toward the mouth,smooth ; 
septa (in the linear portion) very convex, more than their 
diameter apart. 
This very abundant species, first described by Prof. Forbes, 
seems to be distinct from any previously published; the ex- 
pansion near the mouth is very decided. Total length 7 or 8 
inches. Mr Sharpe has fine specimens of this, and there are 
very good ones in the collection of the Geological Survey. The 
reference of these thin shelled species to Pteropod Mollusca, 
which they much resemble, is rendered unnecessary by the 
occurrence of septa, frequently crushed flat, but sometimes 
distinctly visible, in the smaller part of the tubes. 
cated septa are frequently found imbedded by the side of the 
shells in the fine mud; they are extremely convex, and there- | 
fore, as Mr J. Sowerby suggests to me, the more easily crushed 
flat within the shell :—septa placed more at right angles to the 
tube would be more visible, because they would better resist 
pressure. 
Loe.—Quite abundant in the Wenlock shales of Denbigh- 
shire, and also of Montgomeryshire. 
The dislo- | 
vii 
O. venrricosus, Sharpe Sp. 
Syn.—Creseis ventricosa, Sharpe, Geol. Journ. Vol. IT, t.18. 
Sig. 3. 
Sp. Ch.—Subcarinate on the back ? conical, lines of growth 
very oblique, rather prominent. 
A common species, not above 2 or 3 inches long, found with 
the preceding, in the lower parts of the shales of Denbigh- 
shire. 
O. Isex, Sow. 
Syn.—O. annulatus, His. Leth. Suec. Pl. 9. fig. 8% O. Ibex, 
Sow. Sil. Syst t. 5. fig. 80. O. articulatus, ib. fig. 81. O. 
Ibex, Salter, Mem. Geol. Surv. Vol. II. Pt. 1. p. 355. 
Sp. Ch.—Very gradually and irregularly tapering, covered 
with close elevated longitudinal threads, and with oblique 
straight rings, one-fourth or one-fifth the diameter distant in 
the old portion, one-third or half in the young ; section ellip- 
tical; septa oblique in the direction of the rings (distant the 
space of two 2); siphunele a little excentric, nearer to the upper 
edge of the septum. 
This elegant species wants fresh illustration, as the figure 
given by Mr Sowerby, in the Silurian System, is very imper- 
fect, and does not shew the close longitudinal strie. Fine speci- 
mens inthe Cambridge Museum shew a sudden expansion of 
the older part, which then continues parallel, and the rings are 
not more distant in the old than in the younger portions. 
Specimens which we believe to be of this species shew a septum 
between each two rings; but we cannot insist on this character. 
It is permanently distinguished from O. perelegans, Salt., by the 
longitudinal, not transverse striz. 
Portlock’s O. perannulatum differs in haying the ribs much 
closer, and with a strong sigmoid curve, the shell, too, tapers 
much more rapidly (these comparisons being of course made 
between similar ages of the two). De Verneuil and d’Archiac 
seem to be quite in error about the relation of this species to 
O. annulatum, Sow. See Geol. Trans. 2nd Series, Vol. VI. 
Loc. — Many localities round Kendal in Ludlow rock ; 
Dinas Bran, Llangollen, in Wenlock shale. 
Lirvires PLANORBIFORMIS, Conrad Sp.? 
Syn.—Trocholites planorbiformis, Conrad, in Hall’s Paleont. New 
York, Pl. 84. fig. 3. Nautilus primevus, Salter, Quart. Geol. 
Journ. Vol. I. p. 20. 
Descr.—V olutions covering about half the preceding whorl ; 
the back rounded; the sides gibbous, rounded, and sloping 
| inwards toward the umbilicus; sections of whorl reniform, 
twice as wide as deep; septa gently concave, siphuncle large 
and prominent, a little way from the inner edge ; aperture twice 
as broad as long. Surface with fine lines of growth, which bend 
backwards over the dorsal margin, and crossed (in the young 
shell at least) by faint irregular concentric lines: the aperture 
was occasionally thickened, as indicated by indentations on the 
cast. 
It seems hardly right to give a new name to this very inter- 
esting shell, which if it be not identical with Hall’s figured 
species, is yet a near representative of this American form of 
Lituites, in which the whorls are closely appressed and indent- 
ing each other. Hall describes Conrad’s species as having 
