NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—EIRK. 



epistereom. The functions of such canals, were they covered either 

 by a calcified layer or by a hard, impermeable epidermis would prob- 

 ably at once be destroyed. It seems probable that the exterior of 

 the theca was covered by an integument of some sort, but of such a 

 nature as to render possible communication between the canals and 

 the surrounding medium. 



Be this as it may, it is apparent that a covering of these pores by a 

 secondary deposition of stereom may be looked for at the point of 

 attachment of the cystid or in any portion of the theca that may have 

 been in contact with some foreign substance. Hence, where we find 

 canals opening freely to the exterior we may be sure that at that 

 point the cystid was not attached at the time of its death. It is con- 

 ceivable that pores once closed by a calcified layer might after the 

 detachment of the organism be reopened by a resorption of the cov- 

 ering substance, and so we may explain certain cases described here- 

 after. Bearing these facts in mind, it is interesting closely to examine 

 the figures of Aristocijstis as given by Barrande. Some of the more 

 instructive figures are here reproduced on Plate 1. 



Aristocystis. — Figures 1 and 7 imdoubtedly indicate cementation 

 and show the normal efi^ects of such attachment. Figures 5, 6, 9, 

 and 10 as well as other specimens figured by Barrande illustrate a 

 modification of structure however. It is obvious from the flattened 

 area at the base of the theca that the animal was at one time attached. 

 From the size of the region of contact one may judge that the speci- 

 men was affixed until a late stage in its development. Subsequentl}^, 

 it probably became detached. That we are justified in assuming 

 such to have been the case seems to be shown by the structure of the 

 base. The plates show no signs of secondary stereom, such as would 

 be deposited in case of cementation, and the pores are open. Again, 

 the impress of the object to which the cystid was attached is not 

 clearly defined, as it would be were the animal to have been attached 

 at the time of its death, or for some time preceding. 



Figures 5, 6 represent a case in which attachment was doubtless 

 had at an early stage in the ontogeny of the animal. Subsequently 

 an eleutherozoic habit seems to have been maintained. There is 

 no basal impress or flattening to indicate that the cystid was at any 

 time affixed to the bottom, but a slight asymmetry and the presence 

 of a definite apical plate point to this conclusion. 



Under the name Aristocystitesf subcijlindricus, which he gives as a 

 variety of A. hoJiemicus, Barrande figures a number of specimens 

 which seem in no case to have been affixed. Barrande noted the 

 uniform absence of basal depressions and wrote as foUows regarding 

 the material in the explanation of Plate 13: 



Les divers specimens, que nous avons figures sous ce nom, semblent se distinguer, 

 d'abord par leur forme allong^e, arrondie au bout, et ensuite par la disposition des 



