16 



UINTACRINUS: ITS STRUCTURE AND RELATIONS. 



The most rapid decrease in size of the arm is in the first 15 cm. The 

 average rate of taper for the last 85 cm. is .25 mm. in width for every 

 8.5 cm. in length of arm, — or 1 to 340 mm. I have not been able to trace 

 any of these long arms from the calyx to their distal extremity. I have, 

 however, several terminal parts of arms preserved nearly to the end, and I 

 find that as a rule they taper down to a width of about .50 mm. If we 

 suppose the average taper of the above mentioned arm beyond the first 

 15 cm. to continue to the end, it would give a total length of 135 cm. It 

 would seem, however, that the taper is a little more rapid towards the end, 

 as the following measurements of three other specimens will show. The 

 first is an arm branch of 83 cm. long, not traceable to its calyx ; the second 

 and third are parts of arm branches near the distal end : — 



Taking the average of the taper for the upper part of the arm at about 

 1 to 250, it would add about 25 cm. to the length of arm of specimen a on 

 Slab VIII, making it 125 cm., thus giving a total spread of arms of 250 cm., 

 or 8 feet 4 inches, — a size far exceeding that of any other known Crinoid, 

 recent or fossil. 



