ORTHOCERATID^. 
'25 
towards the summit of the cone occupied by the aperture of the 
siphuncle. The stride do not quite reach the siphuncle, a smooth 
area of very limited extent surrounding the latter. 
There yet remains another feature to be described in the 
terminal cap {calotte terminale) of the shell. M. Barrande found 
a few specimens showing spaces partly filled up between the 
concentric striae, and these presented a punctate appearance. 
When the spaces were completely filled, a smooth surface was 
produced, harmonizing with the rest of the shell. The smoothness 
begins to be apparent in the space surrounding the siphuncle, and 
gradually spreads up to the base of the terminal cap {calotte ter- 
Diinale), that is, up to the first suture. The truncation and subse- 
quent reparation of specimens in this condition would be quite 
unsuspected. 
It is obvious that all the cases of truncation of the septa in the 
fossil Cephalopoda cannot be proved to have been produced by 
natural causes, because we have no means of distinguishing between 
the etfects of normal truncation and those of accidental fractures, 
unless the truncated extremity be repaired, as in the species before 
us, by means of a shelly secretion. 
The forms in which periodical truncation of the shell has been 
satisfactorily made out are Ortlioceras^ Gomi>hoceras^ Ascoceras, and 
Glossoceras. 
Detached specimens of the terminal cap of 0. trancatum are by 
no means uncommon in the beds containing that species in 
Bohemia. One of these fragments, figured by Barrande {loc. cit. 
pi. cccxli. ff. 15, 16), measures three inches in its longer diameter, 
and exhibits upon its surface the concentric stride above described h 
Orthoceras trancatum makes its first appearance in the Bohemian 
beds in “ Bande cl 5” (= Middle Bala), where it is rare, but in 
bandes e 1 and e 2 it is very abundant. 
To this species I have assigned two very interesting specimens 
from the Wenlock of Dudley. These consist each of two or three 
septa showing the conical terminal cap with its characteristic 
ornaments admirably preserved. 
Horizon. Etage E, bande e 2 ( = Salopian). Calcaires ampe- 
liteux ( = Third Fauna of Barrande). Wenlock. 
^ Foi’ a very full account of all the phenomena connected with the trunca- 
tion of the septa of 0. truncatuni and other species see Syst. ^1. de la Boheme, 
1877, vol. ii. Texte iv. chap. viii. p. 291 — “ Troncature normale ou periodique 
de la coquille dans certains Cephalopodes paleozoiques ” — upon which the 
foregoing observations are based. 
