CANADIAN FOSSILS. 83 



in the centre of the upper side, is about two lines in diameter, and 

 apparently five-sided. The other aperture between the rays consists 

 of a space covered with plates much smaller than the average size ; 

 these form a small elevation, which is imperfect in all the specimens 

 I have seen, but enough remains to render it almost certain that 

 there was an aperture of some kind in this place. 



The ambulacral areas are five in number, radiating from the mouth, 

 precisely like those of a common star-fish, and composed of two 

 series of oblong plates which alternate with each other in the centre 

 of the furrow. There are about ten of these plates to five lines in 

 length, on each side of the ambulacrum. The pores pass between 

 the plates, two being situated between each two. The ambulacra 

 are three lines wide at the mouth, and about an incb and a half in 

 length in full-grown specimens. As they recede from the centre 

 they curve round towards the right in some specimens, and towards 

 the left in others. 



The mouth appears to be composed often plates ; five of these are 

 at the ends of the ambulacra, and the other five placed in the angles 

 between the ambulacra. In some of the specimens the plates are all 

 smooth, in others covered with small tubercles. 



All the specimens that I have collected were seated upon the rock 

 vnth the mouth upwards, and apparently somewhat flattened by 

 pressure. It is probable that when perfect they were more globular 

 than they are present. One specimen is detached, and shows that 

 the plates covered the whole of the under surface, except a small 

 space in the centre which appears to be without plates ; perhaps 

 this was the point of attachment ; I see no evidence of a column. 



Dedicated to Dr. Bigsby, one of the most able of the first explorers 

 of the geology of this country. 



I regret, that, in consequence of mistaking the meaning of Prof. 

 E. Forbes' remarks on the genus Agelacrinkes in his memoir on the 

 British Cystideae, I supposed this to be the specimen discovered by 

 Dr. Bigsby, and accordingly gave it his name. Since then I have 

 seen Dr. Bigsby's specimen, and find it to be A. Dicksoni. It is too 

 late now to change the names. 



Explanation of Figures. Plate VIII. 

 Figure 1. — Upper side of a specimen partly embedded in stone. 



la. — A polished section through the mouth and two of the grooves, showing 



that the pores penetrate through. 

 2. — Fragment of a crushed specimen, showing two of the rays. 

 2a. — A portion of fig. 2 enlarged to show the pores. 



Locality and Formation. — City of Ottawa. Trenton limestone. 

 Collector. — E. Billings. 



