8 Trans. Acad. Sci. of St. Louis. 



ence in the size of the individual plates, in the number of 

 plates in a column and in the number of columns? in two 

 specimens of about the same size. 



In the examination of the large material at my command, I 

 have become fully convinced that the number of columns, the 

 number of plates composing a column and the size of the in- 

 dividual plates are such variable features that they cannot be 

 considered of primary specific importance. Of much greater 

 value are the ornamentation of the plates, the general shape 

 of the specimen and the number of columns in the ambulac- 

 rum. The ambulacral areas are much more important struc- 

 turally, as they always show a definite number of columns, 

 which forms a very good basis for generic separation. The 

 sole object of the interambulacral areas, as of the interradials 

 in Crinoideae, seems to be to till out space. For this reason 

 interambulacral plates should be considered of secondary 

 importance. 



Having discussed the interambulacral yjlate arrangement, I 

 beg now to call the reader's attention for a few moments to 

 the ornamentation of the plates, which is a very important 

 feature in classification. All species belonging to the 3IeIom- 

 tidae are covered evenly with small short spines, which in 

 some cases show surface ornamentations. Spines are a very 

 good feature for specific distinction. In Melonites multijoorus 

 the spines of the interambulacra and those of the ambulacra 

 are about the same size, while in Melonites crassus, those of 

 the interambulacra are only about half as long as those of the 

 ambulacra. This feature gives the specimen quite a charac- 

 teristic appearance. If the difference in the length of the 

 spines were the result of erosion, as Jackson and Jaggar* 

 claim, we should find the same thing in Melonites multiporiis , 

 where, however, the spines of both fields are always practically 

 one size. After examining the type specimen of Melonites 

 crassus from which Dr. Hambach's figure f was made, and 

 after a very careful comparison with numerous specimens of 

 Melonites multiporus, I am fully convinced that the professor 



* Bull. Amer. Geol. Soc, vol. 7, p. 133. 



t Trans. St. Louis Acad. Sci., vol. 4, PI. C, Fig. 1. 



