314 BRITISH PALZAZOZOIC FOSSILS. [CepHALopopa. 
slightly oblique ; siphon moderate, subcylindrical, nearly central; surface covered with fine, rigid, nearly equal, 
longitudinal strize, nearly twice their thickness apart, separated by flat spaces, about eight in one line. 
Position and Locality—Green, Upper Ludlow, quartzites of Lambrigg Fell, and Laverock Lane, Kendal, 
Westmoreland; whitish tilestone of Storm Hill, Llandeilo, Caermarthenshire ; ‘flags of Llyn Alwen, Den- 
bighshire, the latter being a small fragment with closer and finer strice than usual. 
ORTHOCERAS CENTRALE? (//is.) 
Ref —His. Leth. Suece. t. 9. f. 4. 
Sp. Ch.—Conical, tapering at the rate of one line in one inch, from a diameter of six lines; section circular ; 
siphon central, small; surface girt with sharp, slightly irregular strize, eight or nine in two lines at a diameter 
of six lines, eighteen in same space at a diameter of one and half lines, separated by concave spaces slightly 
exceeding their thickness, with a fine parallel lineation under the lens. 
This species resembles the Devonian 0. striolaris (V. Meyer) in the occasional irregularities and junction 
of the strize, but tapers much more gradually. 
Position and Locality.—Not uncommon in the flaggy Bala limestone of Llandeilo, Caermarthenshire. 
ORTHOCERAS DIMIDIATUM (Sov.) 
Ref. and Syn.—Sil. Syst. t. 8. f. 18 (Mala) = O. subdimidiatum WOrb. Prod. p. 28. 
Sp. Ch.—Slender, tapering at the rate of about half a line in half an inch from a diameter of two lines 
and one-third; section broad-oval; one of the convex sides smooth, the other girt (scarcely half round) by 
thick, equal, slightly oblique ridges, five or six in one line, less than their thickness apart. 
Position and Locality —Rare in the Upper Ludlow rock of Brigsteer, and calcareous Upper Ludlow flags 
of Derby Arms, near Kendal, Westmoreland. One very large specimen, just obtained, imperfect at each end, 
two inches four lines long, and four lines wide at large end, tapers two lines in two inches; and at the larger 
end has the semi-rings varying from four to five in two lines ;—from the Lower Ludlow flags of Leintwardine, 
Shropshire. 
ORTHOCERAS FILOSUM (Sov.) 
Ref.—Sil. Syst. t. 9. f. 3. 
Sp. Ch.—Gradually tapering, length one foot seven inches, tapering at the rate of three lines in two 
inches, from three inches in diameter (slightly less towards the mouth, and slightly more towards the apex) ; 
septa at three and half inches in diameter, rather less than four lines apart, six lines apart at two and a 
quarter inches diameter ; surface marked with sharply-defined, narrow, obtuse, longitudinal ridges, about the 
fifth of a line thick, separated by flat spaces varying in width from half a line to one line, destitute of trans- 
verse strize, but sometimes shewing traces of longitudinal furrows; sometimes every fifth or sixth ridge is a little 
larger than the rest ; section broad-oval; siphon rather large, very slightly eccentric. 
The width at mouth, of the large individuals mentioned above, is about three inches eight lines, the width 
of the smaller end being about eleven lines, the extremity having the appearance of decay and gradual 
crumbling off during the life of the animal. Fragments of this species are distinguished from portions of the 
comparatively small O. angulatum by the narrow, distinctly defined ridges, with the flat interspaces, the absence 
of the minute transverse striation, and the occasional appearance of longitudinal intermediate furrows: when 
the septa are visible they remove all difficulty, as at the same diameter they are double the distance apart in 
the former that they are in the latter species. 
Position and Locality—Common in Lower Ludlow mudstone of Green Quarry, Leintwardine, Shropshire, 
and Garden Quarry, Aymestry, Herefordshire; in the Upper Bala flags of Coldwell, Westmoreland ; black 
Bala shale of Builth Bridge, Radnorshire ; Bala limestone of Coniston Water-head, Lancashire. 
