FOSSILS OF THE SILURIAN ANI> DEVONIAN ROCKS. 189 



Genus Trochonema. saitor. 



Etymology : trochos, a wheel ; nema, a thread. 



This genus is never defined by any writer in this country. It includes spiral 

 shells, having some relations to Pleurotpmaria, but differing from that genus 

 by exterior characters. 



Trochonema fatua. HALL. 



Trochonema fatua, Hall. 20th Reg. Rep., page 3451867. 



Spire elevated ; shell turritiform, consisting of about four or five volutions, 

 which gradually increase to the last one, which is moderately ventricose ; volu- 

 tions biangular, leaving a flattened space upon the back about equal to flattened 

 space between upper angle and suture line ; lower half of last volution 

 rounded ; aperture ovate-elongate. The specimens from which this descrip- 

 tion was made by Prof. Hall were casts, and so is the one before me, which 

 answers in every particular the above description. Prof. Hall states that the 

 surface of the shell, as seen in an imprint in the rock, is finely striated, and 

 that the flattened space on the back of volution is margined on each side by a 

 slender carina, and is covered by coarser striae. 



The size of different specimens varies greatly ; its height is from one to one 

 and seven-eighths of an inch, while the diameter of its base is about four- 

 fifths of its height. 



Formation and Locality. Found in the Niagara rocks, in the quarries east of the city of Louis- 

 ville, Ky. It is very rare; the only specimen known to me belongs to the collection of Major "Win. J. 

 Davis, of Louisville, Ky. 



Trochonema rectilatera. HALL. 



Plate XX., figures 1 and 2. 



Trochonema redilatera, Hall. 24th Regent's Rep., p. 1931872. 

 Trochonema rectilatera, Hall. 27th Regent's Rep., pi. 131875. 



Shell of medium size, turbinate ; breadth and height almost equal ; volution 

 about five, carinated above with straight, nearly vertical sides ; outer one ven- 

 tricose, with two carinae having a wide, vertical, slightly concave space be- 

 tween, which occupies more than one-third the height of the volution. Upper 

 side of the volutions convex for half the distance to the carina, and below this 

 they are concave, giving the form of an ogee. 



In another specimen, apparently the upper [side of the volutions are slightly 

 concave, and regularly sloping downward from the suture to the carina. Lower 

 side of the volution not carinate ; umbilicus small, or closed with a callosity 



