Grabatt — Biserial Ann in Certain Crinoids. 293 



An arm of a somewhat younger individual in the collection 

 of the Boston Society of Natural History is illustrated in ^^, 

 2. Here, normal biserial conditions continue only to plate 49, 

 after which the plates are wedged out, extending all the way 

 across in plate 53. Plate 60, the last one preserved, approaches 

 closely to primitive rectangular outline. Here there are at 

 least ten uniserial plates, the upper five of which are without 

 median tubercles. Compared with fig. la this arm is much 

 less developed, having nearly thirty-five fewer biserial plates. 

 This is readily accounted for if we consider specimen fig. 2 as 

 a younger individual, which appears to be indeed the case 

 from the size of the two specimens. A number of specimens 

 of this species in the collection of Rochester University show 

 uniserial conditions at the apex. 



In an immature individual, in the left arm of a pair, the 

 76th arm plate is wedge-shaped and extends entirely across the 

 arm. Rectangular plates are not preserved. In the adjoining 

 right arm, which is less accelerated, the 73d and 74th plates 

 are wedge-shaped, extending entirely across the arm, while the 

 75 til plate is a truncated wedge. 



In an older specimen (left arm) the 89th plate is wedge- 

 shaped but does not extend entirely across. The 90th, how- 

 ever, does extend all the w^ay across the arm and is the lowest 

 uniserial plate at the apex of the arm. The 92d plate is still 

 truncated, while the 94th is broadly truncated. 



In the youngest individual yet seen (left arm) the 7th is the 

 first biserial plate, as in adults generally. The 43d plate, how- 

 ever, is still wedge-shaped and uniserial. The 44th plate is 

 slightly truncate, the 45th more so, while the terminal plate is 

 rectangular. 



A somewhat abnormal individual (left arm) has five normal 

 rectangular plates above the two oblique ones at the base. 

 The 8th plate still extends across but is wedge-like. The 9tli 

 hardly extends across, being still more wedge-shaped. From 

 the 67th plate upwards the plates are wedge-shaped and uni- 

 serial, extending entirely across. 



In the various species of Encrinus lililformis examined we 

 have thus found the lowest apical plate in which uniseriality is 

 retained to be the following : 



Specimen No. 



1st arm. 



2d an 



4. (Univ. of Rochester) 



89 





1. (Harvard Univ.) 



85 



81 



3. (Univ. of Rochester) 



76 



73 



0. ( - " ) 



67 





2. (Best. See. Nat. Hist.) 



52 





5. (Univ. of Rochester) 



48 





