Swedish Cambrian-Siberian H. and C. 435 
grounded and belong under Hyolithus, (Camerotheca, Diplotheca, 
Matt. Pharetrella Hall); others probably have no relation to Hyo- 
lithus (Coleoprion, Sandb. Coleolus, Hall and Clathrocelia Hall) ; 
still others are grounded on objects whose structure has been mis- 
apprehended (as Hemiceras,! Kichw, and Hyolithellus Bill.) ° 
HYOLITHIDA 
Holmes describes the genus Hyolithus* very fully, as follows: 
Shell bilaterally, symmetrical, pyramidal, or conical, more or less 
elongated, straight or slightly bent in a symmetrical plane, rarely 
towards one of the sides. Cross section usually sub-triangular, 
but also circular, elliptical or lenticular. Dorsal and ventral sides 
usually distinguishable.* Dorsal side slightly arched, flat or gut- 
ter-shaped. Ventral side strongly arched, generally more or less 
angulated along the middle. Mouth angulated or straight; in the 
first case with dorsal side semicircularly arched; in the latter, on 
the other hand, cut off straight or oblique against the dorsal or 
ventral side. The sculpture consists of growth lines parallel to the 
orifice, occasionally besides of longitudinal elevated lines or 
mouldings, whereby more or less complicated sculptured surfaces can 
arise. Diaphragms are often observed in the apex of the shell ; 
they are entire, not perforated by any siphon. The operculum 
completely closed the mouth, no matter whether the same was an- 
gulated or straight; sometimes it was slightly conical, with the 
_ nucleus nearest the ventralside, and with concentric growth lines. 
1“ Hemiceras is evidently grounded on the interior of siphons of Endoceras, 
as plainly appears from Hichwald’s figures of all three species.”’ 
2 Speaking of the family Hyolithellidee of Walcott, Holm says ‘“The name is 
quite inapplicable * * * since the genus Ayolithellus Bill. is grounded chiefly 
on a Brachi»pod previously described by Hall under the genericname Discineila 
which Billings and Walcott wrongly declared to be the operculum of a form 
nearly related to Hyolithus. This was, by the last named authors, placed to- 
gether with some shining tubular form, whose nature is hard to determine.”’ 
° He adopts this spelling as the correct form of the name, though the origina- 
tor of it, Hichwald, wrote Hyolithes. 
*“Opinions have changed as to which side should be regarded as the dorsal, 
and which the ventral. In the simplest forms both sides are quite obscurely dif- 
ferentiated, but with the more highly developed genus (subgenus Hyolithus sens. 
strict), the separation on the other hand is plain. With the latter two opposite 
sides can always be distinguished,they are shown by amoreor less sharp edge. One 
of these sides is longer and has the edge of the orifice strongly arched forwards, and 
this is considered the dorsal side ; the other whose mouth-edge is transverse, as 
the ventral. Same conception has been entertained by Salter, Matthew, and 
from 1886 by Walcott. But Hall, Billings and Walcott, before 1886, have held 
the contrary view. Barrande avoided distinctly determining this point by apply- 
ing the terms “‘La grande face,’ (dorsal side) and “Les petitis faces (ventral 
side). Novak named them “Die Hinderflache,”’? and “Die Vorderflache. By fol- 
lowing the development from the higher stages backward to the lower, one can 
decide even in the earliest form which side is dorsal and which ventral in the 
sub-genus Hyolithus sens. str.’ 
