60 
NATJTILOIDEA. 
ff. 15-19, ? pi. cccxxxvi. fF. 17-21, 30-32, ? pi. cccxcix. IT. 20-22, 
pi. ccccxlv. IF. 6-8, pi. ccccli. IF. 4-11, pi. cccclx. ff. 4-10. 
1870. OrtJioceras subannulare, Barrande, Bistrib. dea C^ph. dans lea 
Contrees Siluriennes, p. 50 (Extrait du Syst. Sil. de la Boheme, 
vol. iii. 4me ser. pis. cccli. a cccclx.). 
1873. Orthoceras subannulare, Salter, Cat. Camb. and Sil. Foss. p. 159. 
1876. Orthoceras subannulare, Tromelin & Lebesconte, Assoc. Fran 9 . 
pour I’avancement des Sciences, 4me sess. (Nantes), p. 621, Tableau 
D, numero 33. 
1882. Orthoceras subannulare, Blake, Britisb Foss. Cepb. pt. i. p. 94, 
pi. V. ff. 6, 6 a. 
Sp. Char. Shell straight, or very slightly curved in some speci- 
mens. The section is always circular. Bate of increase about 1 
in 10. Body-chamber not known in the adult, but in an immature 
individual it measured four times the diameter of the base, or about 
h the total length of the shell. The septa are widely separated, 
having a maximum width of about | that of the shell. The 
siphuncle is generally a little eccentric, part of it, however, always 
touching the central axis. The surface of the shell is variable, the 
ornaments consisting mainly of more or less prominent rounded 
sinuous annulations upon the greater part of the shell ; their 
direction is oblique about 12°. The surface of the test is covered 
with very regularly disposed transverse striae, imbricating upwards ; 
about ten of them can generally be counted from summit to summit 
of the annulations. 
Uemarlcs. This species was founded by Miinster upon very imper- 
fect specimens, and we must therefore turu to the large series figured 
by Barrande, and to the Bohemian and other examples in the 
National Collection, to supply the material for its more complete study. 
We learn from these that the species is liable to considerable 
variation in its surface characters. While in some cases the undu- 
lations are well-marked, in others they almost totally disappear, 
though there are not wanting transitional forms connecting tliese 
two extremes. 
Examples showing the absence of undulations are figured by 
Barrande (pi. ccccli. ff. 4, 8), and there are also specimens in the 
National Collection in which the undulations are extremely feeble 
The typical form, as desciibed and figured by Munster, was marked 
by flattish undulations, and the British examples are similarly 
characterized, the undulations being by no means so strong as they 
are in some Bohemian examples, which make an approach towards 
0. dulce in the sharpness auff prominence of their rings. M^Coy 
gives us important testimony as regards the identity of the British 
species with Munster’s ; he says, “ On comparison with authentic 
