Summit Structure of Pentremites. 491 



ries, even to the addition of a greater than the normal 

 number of plates. It should, however, be remarked, that 

 each branch of the proboscis, in both these instances, is 

 as perfectly and regularly formed as any single proboscis. 



Whether this division of the proboscis is normal or ab- 

 normal, we may be sure that one of the branches was the 

 buccal and the other the anal tube ; but which performed 

 the first, and which tlie latter office, there seems to be no 

 means of ascertaining. 



Two instances have been observed of a small tumid 

 projection on the side of the proboscis of Actinocrinus, 

 near the upper end, above the reach of the arms ; which 

 has suggested the idea that an aperture at this point, now 

 closed, connected with the anal canal ; however, these 

 may be only obstructions of the tube. 



Instances showing the recuperative power of crinoids. 



The remarkable recuperative power of the crinoidea is 

 shown in the following instances, observed among the 

 collections at Burlington. 



1. Two specimens of Pentremites Norwoodii have lost 

 each an entire side, leaving the bodies quadrangular, but 

 scarcely affecting their symmetry, and what is most re- 

 markable, one of the pseudambulacral fields of each speci- 

 men contains two complete sets of poral and supplemen- 

 tary poral pieces. The specimens are of full average 

 size, notwithstanding their loss, from which they seem to 

 have suffered little inconvenience. 



2. Three calyces of Platycrinus of different species are 

 found to have lost each a first radial. The calyces are 

 quite symmetrical, and composed of only four plates each, 

 resting on a quadrangular base. 



3. Two specimens of Actinocrinus are found with an 

 entire ray wanting in each ; the remaining parts forming 

 a perfectly symmetrical body of only four rays, which 



