50 CANADIAN FOSSILS. 



Heterocrinus tenuis, Billings. 

 Plate IV. Figures 6a-6b. 



(H. TENUIS, Report Geological Survey of Canada, 1856, page 2*73.) 



Description. — Much smaller than H. simplex; arms, long, very 

 slender, and several times divided ; column, very obscurely penta- 

 gonal, composed of sub-globular joints ; proboscis, extending nearly 

 to the apices of the arms ; length, including the arms, from ten to 

 sixteen lines ; without the arms, from one and a half to two and 

 a half lines ; diameter at base of arms, about two lines ; of column, 

 at base of cup, half a line. 



It is not certain that this species should be referred to the genus 

 Heterocrinus. The plates of all the specimens in the collection are 

 so closely united that their number and arrangement cannot be 

 satisfactorily made out. The weight of the evidence is in favor of 

 the genus under which I have placed it. The species, when several 

 times attentively examined, is easily distinguished from H. simplex. 

 In that species the column, for a short distance below the cup, is 

 smooth and slender, and it enlarges suddenly from a few lines below, 

 until it forms rather a broad base for the pelvis to stand upon. But 

 in H. tenuis the column continues moniliform to the base of the cup 

 and without enlarging, but on the contrary is rather less in diameter 

 at the point of contact than it is below. In one specimen there are 

 forty two joints in the first nine lines from the pelvis, and some 

 irregularities in the size can be seen. They are thinner near the cup 

 and gradually become thicker, so that at two inches from the pelvis 

 there are only sixteen in half an inch. The arms, although much 

 more slender than those of H. simplex, usually lie folded together, or 

 but slightly separated. 



Explanation op Fiqorks. Plate IV. 



Figure 6a. A specimen with part of the column. 

 " 66. A few joints of the column, enlarged. 



Locality and formation. — Trenton limestone, Ottawa and Montreal. 



