1886.] NATURAL SCIENCES OF PHILADELPHIA. 18T 



Detached plates of Agassizocrinus are found in great abundance 

 throughout the Kaskaskia group, but perfect calices are extremely 

 rare. The cause of this is perhaps explained by the mode in 

 which the different plates are'united with one another. Only the 

 underbasals seem to have been united by common suture, and 

 these, as a rule, are the onlj^ plates which are not found discon- 

 nected in the fossil. The apposed faces of the succeeding plates 

 are vei'tically and horizontally provided with rather conspicuous 

 ramifying furrows, which enter the surface of the plates (PL 5, 

 fig. 17). The ramifications vary somewhat, even among corres- 

 ponding plates, but those of apposed faces correspond with each 

 other, and together form small tunnels, which branch once or 

 twice, grrowino- narrower at their outer ends. Between basals and 

 underbasals there are generally three of these furrows to each 

 plate ; but they unite to one on entering the underbasal cavity, 

 where they seem to communicate with certain pits or openings, 

 which have been overlooked by previous writers. 



The underbasals form an almost solid disk, with a shallow, cup- 

 shaped inner cavity. The latter contains six deep pits or cham- 

 bers, closed at the bottom, a central one surrounded by five others. 

 These pits either contained the lower part of the chambered 

 organ, or they indicate where the vessels extended down from 

 the chambers into the larval stem. It is possible that the axial 

 canals which descend from the body cavity were lodged in these 

 chambers, together with the vessels of the latter, and this might 

 explain the apparent communication of the pits with the radiat- 

 ing furrows above mentioned. But it may be possible, also, that 

 the furrows lodged groups of ligamentous fibres, by which the 

 union of the plates was rendered firmer and closer than it would 

 be when only two smooth surfaces were apposed. The plates, 

 however, as a general rule, are found to be disconnected, and the 

 radials at their lateral faces possess fossae, thus proving there 

 existed a certain degree of mobility between the calyx plates 

 generally. Similar ramifying furrows were discovered by us in 

 the genus BaryctHnus between the apposed articular facets of 

 radials and brachials, plates which evidently were not united by 

 a close suture. 



Agassizocrinus in the earlier stages was attached by a column, 

 but led a free life later on, when the scar to which the stem had 

 been attached was gradually obliterated, so as to leave no traces 

 behind. The secretion of calcareous matter extended over the 



