NO. 1846. ON CERTAIN ELEUTHEROZOTC PELMATOZOA—EIRK. 107 



entire column of the Crinoidea is quite oroundless. Nor may the 

 centrale be compared with the apical plate of the Echinoidea, thereby 

 giving a footing to far-fetched theories of mutual interrelationships. 



TYPE 3. 



Agassizocrinus. — ^The sole genus referable to this division is Agas- 

 sizocrinus, which flourished in enormous numbers in the late Missis- 

 sippian seas. The structure characteristic of this group is the close 

 union and subsequent fusion of the proximal circlet of plates. No 

 centrale was formetl, apparently. As elsewhere noted Saccocoma 

 may possibly belong in this division, for in that genus the ''centrale" 

 may simply represent the fused basals. 



The infrabasals in Agassizocrinus are large and massive, while the 

 axial canal is comparatively small. Upon detachment from the 

 column a filling of the axial canal probably took place, but the result- 

 ant plug must of necessity have been insignificant in size, and may 

 indeed never have reached the surface. At any rate no trace of the 

 plug can be seen externally. A very close union was effected between 

 the infrabasals, and in the majority of cases complete fusion took 

 place in the adult stages. As a rule the fusion of the plates was 

 followed by a secondary deposition of stereom over the entire surf ace 

 of the infrabasal knob, which efi'ectually obliterates all traces of 

 sutures. In the figure here given (PI. 10, fig. 5), it is to be noted that 

 the infrabasal sutures are indicated. This is somewhat unusual for 

 the genus. These fused infrabasals are preserved to us in great 

 numbers, certain beds bemg largely composed of them associated with 

 disarticulated calj^cal plates. 



Agassizocrinus is particularly interesting because of the fact that 

 the acquisition of an eleutherozoic habit is a comparatively new thing, 

 and occurs in a line where such disruption indicates a wide deviation 

 from the normal. In spite of this fact the innovation seems to have 

 been a most successful one, judging from the prolific development of 

 the genus. Agassizocrinus appears to have been near the termination 

 of its genetic Ime, and it is perhaps on this account alone that it has 

 not given rise to a Ime of eleutherozoic descendants. 



As to the habits of Agassizocrinus little may positively be said. It 

 seems highly probable that an eleutherozoic existence was maintained 

 from a very early ontogenetic stage, for no trace of a stem cicatrix is 

 to be seen. Considering the very heavy nature of the cup plates and 

 the comparatively massive and short arms it scarcelj^ is to be held 

 that the animal was a free-swimming organism. Rather, I think it 

 should be considered that Agassizocrinus for the greater part of the 

 time crawled about on the bottom and was capable of swimming but 

 short distances. 



