638 T. ROBERTS ON A CONOCERAS FROM THE 
near the margin and possessing a radial structure, and (ii) a kind 
of cylinder, which is placed at the margin immediately beneath the 
curving backward of the septa. Kayser regards the peculiar cha- 
racter of the septa as a malformation due to a diseased development 
of the animal. 
This fossil has many characters in common with Conoceras :— 
_(a) the bending of the sutures of the septa forming a cheyron-band, 
but in this Gomphoceras the apex of the chevron is directed back- 
wards ; (6) the longitudinal groove uniting the apices of the depres- 
sions is similar to what occurs in some specimens of Conoceras 
from Bohemia *; (c) the structure placed beneath the chevron-band 
appears to be much the same in both forms. What the subcentral 
structure (siphuncle of Kayser) may be it is difficult to decide. On 
the outside of the last three chambers of the Gomphoceras there is 
present a structure showing a radial arrangement of parts, and 
apparently occupyixg the same relative position to the chevron- 
band as that of the so-called siphuncle in the middle portion of the 
fossil, so that they seem to be continuous; if so, then Kayser’s 
determination of its being the siphuncle is incorrect, as that which 
he believes to be the siphuncle has a subcentral position in one 
part of the fossil, and seems, in another part, to cut the shell itself 
obliquely, and must therefore be a foreign body. 
The only English species as yet recorded is Conoceras eoum, in 
which the forward inclination of the septa is only feebly marked. 
It is doubtfully referred to this subgenus by Blake fy. 
Conoceras angulosum and Bathmoceras Linnarsoni differ from all 
the other known species by their sharply tapering shell. 
To Bathmoceras preposterum and B. complexum the Llanvirn 
species presents considerable affinity. I have compared it with a 
specimen of B. preposterum from Bohemia, in the collection made 
by Mr. Marr, now in the Woodwardian Museum, and find that it 
agrees with it in most of its characters. Barrande distinguishes 
his two Bohemian species by the manner in which he considers that 
the septa are developed {; in B. preposterum the septa first appear 
at the sides of the siphuncle, whilst in B. complexum they are first 
formed on the dorsal side, or that opposite to the siphuncle; it is 
thus he accounts for the usual incompleteness of the last two or 
three septa. Blake points out that this character is of no value, 
“since many Orthocerata of ordinary character, Phragmocerata, 
and even Ammonites show similar incomplete septa, dependent pro- 
bably on accidents of preservation” §. Barrande himself, too, states 
that in Orthoceras imperficiens, O. styloideum, &c., some of the 
septa are incomplete||. The only distinction which I am able to 
make between the Llanvirn form and those of Bohemia, is the pre- 
sence of corrugations of the shell in our species, and their absence 
* Barrande, Sil. Syst. Bohéme, vol. ii. texte iii. p. 794. 
t Blake, Brit. Foss. Ceph. part i. p. 166. 
+ Barrande, Sil. Syst. Bohéme, vol. ii. texte iii. p. 793. 
§ Blake, Brit. Foss. Ceph. part i. p. 53. 
|| Barrande, Sil. Syst. Bohéme, vol. ii. texte ili. p. 793. 
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