Klem — The Development of Agaricocrinus. 173 



only. If we examine the way in which extra arms arise, the 

 folly of considering the number of arms important becomes 

 still more apparent. In rays having regularly three arms, the 

 third arises from a tertiary radial which cuts off the upper 

 angles of the secondary radials. If this plate should divide 

 vertically, forming two tertiary radials, then an arm would 

 spring from each of these two tertiary radials, and there would 

 be four arms to that ray. Increase in the number of arms 

 seems to be due solely to the dividing and cutting off of the 

 sides of the plates. Injury to some of the plates where arms 

 were to form would prevent the further growth of the arm 

 and leave it undeveloped. Such a case I have shown at Fig. 

 2, where the middle anterior ray started to produce a second 

 arm. The size of the arms and the number of joints are worth- 

 less as features of classification, as they are preserved in 

 comparatively few specimens and represent only the age of 

 the animal. The only way in which the number of joints 

 could be of value is, if there were a certain definite number 

 of joints to the inch for each species. 



The pentamerous arrangement of the parts is the rule 

 throughout the sub-kingdom of the Echinodermata, but we 

 cannot find another division in the whole animal kingdom so 

 subject to abnormal development. In addition to the above 

 examples of abnormity, I will mention a few striking instances 

 of its occurrence in other families, as examples of deviation 

 from the regular pentamerous type are not uncommon. In 

 Bulletin 3 of the Illinois State Museum of Natural History, 

 on page 19, in a comparison between Batocrinus facetus and 

 B. Lyonanus, we find the following statement: *' In Bato- 

 crinus facetus the three-armed series is on the right of the 

 azygous side, in this it is on the left. In that species, there 

 are four regular interradials and eleven azygous plates, in 

 this species there are three regular interradials and six azygous 

 plates." In the same Bulletin the following remarks are 

 found in the description of Zeacrinusbellulus: ** This species 

 bears some resemblance to Gyathocrinus 7naniformis of Yan- 

 dell and Shumard which has generally been referred to Zea- 

 crinus. In that species the subradials are long and abruptly 

 bend into the columnar cavity and upward so as to form a 



