AKCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 287 
conch usually varies between 5.5 and 6.5, but may be as low as 
4 and as high as 7. The siphuncle is relatively large, equalling 
at least 55 to 60 per cent of the diameter of the conch at the 
lower end of the latter, though apparently diminishing in size 
toward its upper end as in Paractinoceras, The sutures of the 
septa curve moderately downward on the ventral side of the 
conch. The surface of the shell usually is smooth, but occasion- 
ally is striated or ribbed transversely in a direction more or less 
parallel to the sutures of the septa. 
17. Actinoceras Sp. 
Plate XXX, figs. 2A, B; Plate XXXII, fig. 7 
The specimen consists of a fragment of a phragmacone 130 
mm. in length, of which the lower part, beneath an oblique crack, 
is not figured. The specimen is strongly flattened on its dorsal 
and ventral sides, probably due to compression of the enclosing 
sediments previous to solidification. In my opinion this com- 
pression of the sediments resulted merely in a shortening of 
their vertical dimensions, and was not accompanied by any 
horizontal expansion, either of the sediments or of their contents. 
If this view be correct, then the present lateral apical angle of 
the specimen and its present width correspond closely to the 
original lateral dimensions of the specimen, though its dorso- 
ventral dimensions are greatly shortened. 
In its present condition the lateral apical angle of the conch 
equals 10 degrees, and from 6 to 6.5 camerae occupy a length 
equal to the lateral diameter of the conch at the top of the series 
of camerae being counted. Along the dorsal side of the conch 
the sutures of the septa curve only slightly downward, possibly 
only I or 2 mm., and even this slight downward curvature may 
be due to compression. Along the ventral side, on the contrary, 
the downward curvature of the sutures is much more pronounced, 
equalling 7 mm. Most of this downward curvature appears to 
take place along the ventro-lateral parts of the conch, the down- 
ward curvature being much more moderate along the median 
parts of the ventral side of the conch. At the top of the speci- 
