64 
KArilLOIDEA. 
former measurement is taken from the last two septa, at the base 
of the chamber, where they are probably closer together than in 
other parts of the shell. The convexity of the septal surface is 
about k the longer diameter of the shell, but in the type it is 
The sij)huncle is usually central, but occasionally a little eccentric. 
In large examples it has a diameter of from 4 to that of the 
shell; it is cylindrical in shape, and has a ver}’ shght expansion 
between the septa. 
The ornaments consist of, first, transverse, undulating, rounded 
or subacute ribs, separated by concave interspaces a little wider 
than the ribs. In uncompressed examples the latter are distant 
from 5 to -I the diameter. Secondly, the whole surface of the shell 
is covered with fimbriated riblets or lamellae, running parallel with 
the ribs. The fimbriae, when perfect, project vertically from the 
surface of the test ; they vary greatly in number and character, 
but are always more crowded upon the ribs than in the interspaces. 
Fine parallel striae may be seen between the fimbnae, on well- 
preserved specimens. Somewhat faintly defined longitudinal eleva- 
tions traverse the surface, crossing the ribs, and sometimes imparting 
to them an obscurely nodose character : these elevations generally 
correspond with the arches of the fimbriae. 
In some examples (including Sowerby's type) the ribs for a short 
interval (equivalent generally to the space occupied by two or three 
ribs) entirely disappear, and their place is filled up by the fimbriae, 
very much crowded together. That these ribless intervals do not 
represent the true aperture is shown by the fact that they are 
succeeded in some examples by a ribbed surface of indefinite length. 
A specimen in the Museum of Practical Geology exhibits very 
distinct bands of colour. This interesting phenomenon has been 
seen also in other species, such as Orthoceras angidiferum^ d’Arch. 
& de Yern.^ In this the colour-bands are chevron-shaped, and have 
a very beautiful and characteristic appearance. 
BemarTcs. The fact of this species having been erroneously as- 
signed by its author to the horizon of the Carboniferous Limestone 
(Derbyshire Peak Limestone), in the “Supplementary Index” to 
vol. ii. of the Min. Conch., has led many to suppose that Sowerby’s 
type of 0. annulatum was a Carboniferous species. But in his 
paper, “ On the Geology of Coalbrook Dale,” ^ Professor Prest- 
wich records it from the Wenlock and other Silurian rocks of 
1 Trans. Geol. Soc. 1842, vol. vi. p. 346, tab. xxvii. f. 6. 
’ Trans. Geol. Soc. 1850, vol. t. pt, 3, p. 422. 
