

An analogy between the development of the plates of crinoids 



and the leaves of Sassafras 



Edwin W. Humphreys 



It is a fact, well known to paleontologists, that the calcareous 

 plates in the arms of crinoids are arranged in a single row in cer- 

 tain species, as in the Pisocrinidae and other members of the order 

 Larviformia, or in a double row, as in Encrimis liliiformis and others. 

 The biserial forms are uniserial at the lower end and here the plates 

 are quadrangular in shape. These latter grade into wedge-shaped 

 plates, which are next followed by biserially arranged and more or 

 less pentagonal-shaped ones. 



It was pointed out not long ago, by Dr. A. W. Grabau,* that in 

 a number of forms, in following on toward the tips, the same series 

 of changes is repeated, but in reversed order, so that the uniserial 

 quadrangular plates are again found at the upper ends. There is 

 then, starting from the base and working upward, or from the tip 

 and proceeding downward, a change in the plates from a uniserial 

 to a biserial arrangement and from a quadrangular, more or less 

 regular outline, to one that is irregularly pentagonal. In other 

 words the plates of simplest form and arrangement are at the two 

 extremities and the more complex ones are between them. Fig- 



ia and lb are reproductions of two of Dr. Grabau's fig- 

 ures illustrating these features. 



From the arm plates of crinoids to the leaves of Sassafras 

 ftiay seem like a far call, but it is an interesting fact that in the leaf 

 arrangement in many of the branchlets of a season's growth, in 

 suckers, and in seedlings of Sassafras, the arrangement of the 

 plates in the biserial crinoid arm is at once suggested. Examina- 

 tion of a large number of specimens collected in Bronx Park has 

 shown none but entire leaves at the bases of all of them, while 

 above these are either or both the bilobed and trilobed ones. The 

 °rder in which these latter two forms appear is not constant, how- 

 ler. Sometimes the bilobed ones are below the trilobed and vice 



URES 



Amer. Jour. Sci. 16 : 289-300. /. /-//. 1903 



571 



