Swedish Cambrian- Siberian H. and (. 437 
The shell with four quite sharp edges, and four sides tlat or slightly 
concave The aperture obliquely cut, with the ventral side longer, 
On the surface of the shell fine longitudinal raised lines prevailing, 
(—Bactrotheca, p.p., Novak). L. Silurian. 
6. Lenticulares. The transverse section almost symmetrically 
lentiform. The lateral edges extraordinarily acute. The lines of 
growth on the dorsal as well as on the ventral side concave. Upper 
Silurian. 
In the sub-genus Hyolithus, as restricted by Holm, he finds two 
main divisions, these are: 
I. Aquidorsati. The ventral side wanting grooved channels near 
the lateral edges. If the lines of growth there change direction 
that is done by degrees with an even curve. The boundary line 
between the real dorsal and the ventral sides are therefore the 
same as the lateral edges of the shell, which are almost always 
sharp. 
II. Magnidorsati. A little from each of the lateral edges on the 
ventral side is a stronger or feebler channel, bordered outward by 
one or several longitudinal raised lines at the side of the channel, 
the growth suddenly changes in direction. Those channels are 
the boundary lines between the real dorsal and the ventral sides, 
which therefore are not identical with the lateral edges of the shell, 
the dorsal side turning along those edges, and passing into the 
ventral half of the shell. What seems to be the ventral side, there- 
fore, is divided into three fields, those belonging to the turned over 
dorsal side, and that in the middle being the real ventral side. 
Under the division quidorsati the following groups are con— 
tained. 
1. Transversistriati. The surface of the dorsal side as well as the 
ventral side has lines of growth only This is a large section con- 
taining species ranging from the Ci#landicus Zone to the Upper 
Silurian (c.) of the Swedish divisions. 
2. Ventrilineatt. The surface of the dorsal side with lines of 
growth only, that of the ventral side, ornamented with coarser or 
finer lines, straight longitudinal raised lines covering the whole 
breadth of surface. The lateral edges acute.—Forchammeri— 
Lituite Zone, mostly Lower Silurian. 
3. Dorsilineati. The dorsal side with longitudinal raised ities 
over the whole surface, or at least at the lateral edges.—Lower 
Cambrian to Trinucleus Zone. 
4. Crispati. The surface of the shell on the dorsal side as well 
as on the ventral side with very elevated lamelliform longitudinal 
raised lines, most often with the edge more or less undulating.— 
Lower Silurian. 
