34 
NAT7TIL0IDEA. 
stronger one, but later on fairly strong ribs are developed, still 
covered by the riblets, and all curving back as in the type. We are 
thus permitted to consider that var. a represents the young form 
and var. /3 the more adult, accounting for the difference of dimen- 
sions by the distortion and compression The septa 
are uniformly concave, but make a backward sinus on the front 
[convex border], and there are 32 per whorl.” 
Itemarlcs. This species is represented in the Collection by a single 
imperfect specimen, in which, however, the acute ribs and the coarse 
striations parallel to them are well shown. This specimen, which 
is from the Ordovician of Norway, is larger than the one figured by 
Blake from the Bala Beds of Sholeshook, measuring nearly 4 inches 
across. The transverse diameter of the outer whorl (which is the 
only one seen) is inches. The ribs form a broad sinus upon the 
periphery. The chambers are filled with crystalline calcite, and the 
siphuncle is not seen. 
Horizon. Etage 5 a (Bala Series). 
Locality. Hero, Norway. 
Trochoceras optatum, Barrande. 
1867. Trochoceras optatum, Barrande, Syst. Sil. de la Boheme, vol. ii, 
pt. i. p. Ill, pi. xxiii. ff. 5-10. 
1877. Trochoceras optatum, Barrande, ibid. Suppl^m. et S^rie tardive, 
p. 87, pi. ccccxcv. ff. 5-8. 
Sp. Char. This is one of the largest species in the Bohemian 
basin. Adult examples do not possess more than two whorls, in 
which the want of symmetry is very apparent, the inner whorl 
having its apical portion elevated above the plane of the outer one 
upon which it rests. The outer whorl frees itself from the inner 
for the distance of about of a whorl, thus leaving a considerable 
space between the two, and resembling in this respect Ophidioceras. 
The transverse section is nearly circular. The shell enlarges regu- 
larly and somewhat rapidly in diameter, the latter increasing about 
four times in a complete volution. The body-chamber occupies that 
part of the last whorl which is free from the preceding one. Its 
capacity is greater than that of the rest of the shell, of which it 
forms nearly one half of the length. The aperture is oval, the ratio 
of the ventro-dorsal diameter to the transverse being as 9 : 8. The 
septa, in the vicinity of the body-chamber (where the shell has a 
diameter of 15 lines), are about -h that diameter distant from each 
other; their convexity about ^ the ventro-dorsal diameter. The 
siphuncle is close to the convex border ; its elements are cylindrical. 
The ornaments consist of oblique, acute, and prominent annulations, 
