436 Canadian Record of Science. 
Shell formed of calcium-carbonate, of the same nature and appear- 
ance as in Gasteropods, 
Dr. Holm says that species of Hyolithus have been described un- 
der the following generic names :—Hyolithes, Eich. Orthoceras, 
Munst., Theca, J. Sow, Pugiunculus, Barr., Vaginella (pars.) d’Orb- 
Cleodora (pars.) Ludw. Cleidotheca (pars.) Centrotheca (pars ) 
Salt. Cryptocaris (pars ) Barr. Camerotheca (pars.) Diplotheca (pars.) 
Matt. Orthotheca (pars.) Novak, Pharetrella (pars.) Hall, Ceratotheca 
(pars.) Bactrotheca (pars ) Novak. 
Although Dr. Holm rejects Orthotheca Novak as a genus he ac- 
cepts the term as of sub-generic value, and divides Hyolithes into 
two subgenera as follows: 
Subgenus 1, OrTHoTHEcA, Novak, 1887. 
Mouth quite transverse, forming one plane, Operculum thereby 
also flat or slight convex, seldom with the nucleus concave, but al- 
ways having the edge of the operculum in one and the same plain. 
The dorsal part of the operculum is never distinctly semiconical. 
Subgenus Hyo.iraus, sensu. str., Eichwald, 1840. 
Edge of the orifice on the dorsal side semi-circular projecting, on 
the ventral side the edge is transverse, therefore the mouth of the 
shell forms two planes coming together at an obtuse angle. Oper- 
culum having the same form as the mouth, and similarly angled, 
and consisting of a small lunate ventral part and a semi-conical 
dorsal part. 
Dr. Holm has been at great pains to arrange systematically the 
forty species of Hyolithes which have passed under his observa- 
tion, and as he has in almost all cases been able to show the exact 
geological horizon from which these species have come, the ar- 
rangement is of great value to the biologist. In his sub-genus 
Orthotheca we find the following sections : : 
1. Teretes. The transverse section circular or almost circular— 
Cambrian (Kjerulfi to Forchammeri Zone). 
2. Complanati. The transverse section perfectly rounded, but 
_ with the dorsal side distinctly, though slightly flattened. Lower 
Cambrian. 
3. Plicati. The transverse section, reniform, cordiform or tri- 
angular, with the dorsal edge of the section concave, the dorsal side 
strongly grooved. Cambrian (Clandicus-Forchammeri Zone). 
4. Semielliptici. The transverse section semi-elliptical or sub- 
trapezoidal. The lateral edges sharp oralmost sharp. The dorsal 
side plain or very slightly grooved. The aperture usually obliquely- 
cut, with the ventral side projecting (—Bactrotheca, p. p., Nov.) 
Lower Silurian. 
5. Quadrangulares. The transverse section almost rectangular. 
