1881.] NATURAL SCIENCES OF PHILADELPHIA. 197 



interlock. This process commences at the extremities of the 

 arras, and gradually involves the lower portions down to the bases. 

 During this stage we often find the lower arm joints quadrangular, 

 with parallel sutures, followed by wedge-form and cuneiform pieces, 

 and finall\- the tips constructed of a double series of plates. 



The same development, which thus took place during the life of 

 the individual, is observed to go on in geological times, but not 

 contemporaneously in different families. In the Actinocrinidffi 

 and Platycrinida? it became complete in the Silurian, and is found 

 invariably in all succeedyig forms. In the Cyathocrinidie, that 

 structure appeared only at the close of the Subcarboniferous 

 shortly before the famih' became extinct. In this group, the arm 

 pieces attained that marked wedge-form which everywhere ])re- 

 ceded the double joints in the Burlington limestone, and here in 

 some species of Poteriocrinus and Cceliocrinus the plates beo-an 

 to interlock already at the tips of the arms. This became more 

 frequent and more conspicuous in the Kaskaskia group, where in 

 some few cases, it extended to the entire arm. 



The ditierent stages of individual growth, as the3' became gradu- 

 ally introduced paleontologically and fixed, undoubtedly form 

 excellent generic characters, but we must not forget that there 

 was a time in the life of the crinoid at which the arms were 

 neither single- nor double-jointed, but at which the joints began 

 to interlock, and when probably a very few daj^s brought about 

 important changes in the arms of the growing animal. This 

 stage is represented paleontologicallj^ among the C3^athocrinid£e 

 b}' Eupachycrinus, Erisocrinus and Hydreionocrinus, and jn this 

 view of the ease it is not difficult to understand how this arm 

 structure may be of generic importance as a rule, but scarcely of 

 specific value in exceptional cases. 



It has been stated that the double-joint structure was introduced 

 in the Spha^roidocrinidaj in the Silurian, and this occurred under 

 exactly the same conditions as it did later on in the Cvathocri- 

 nidse. By far the greater number of species in the Lower Silurian 

 have single arm joints, and these, with a few exceptions, consist of 

 quadrangular pieces with parallel sutures. In the Upper Silurian 

 we find a few forms with single joints, and along with them arms 

 with cuneiform joints — either interlocking or not — associated in 

 the same strata with species having double series of arm plates 

 and we find all intermediate gradations between the two extremes. 



