NO. 1846. ON CERTAIN ELEUTHEROZOIG PELMATOZOA—KIRK. 19 



Such types are Erinocystis, Amygdalocystis, and Cheirocrinus. All 

 these C3^stids may be held as eleutherozoic forms, and the further 

 assumption of a partially perfected prostrate habit rests largely on 

 the evidence of such structural modifications as have hitherto been 

 cited as characterizing such a form of life. 



Amygdalocystis. — Amygdalocystis has an unusually short and feeble 

 column which generally gives evidence of having been looped. The 

 probable proximal ancestor of the genus, which is as yet unde- 

 scribed, is a subcylindrical form with three food grooves. The 

 flattening of the theca and the atrophy of one of the food grooves, 

 as well as the nature of the column, seem to point to the assumption 

 of a prostrate form of life. One has difficulty in explaining such a 

 compression of the theca on any other basis. A spherical or sub- 

 cylindrical type, if an erect position be constantly maintained, mil 

 tend to give rise to a line in which the radial symmetry is made 

 more and more perfect. The almost equal convexity of the two 

 sides, as shown in Amygdalocystis and Platycystis, would seem to 

 indicate at least that the animal did not constantly rest on one 

 side, even were a prostrate condition maintained. 



Erinocystis. — Tliis tendency toward the assumption of a prostrate 

 condition is quite marked in the small group to which Jaekel has 

 given the name Erinocystis. This genus was short-lived and appar- 

 ently left no descendants. Erinocystis illustrates modifications 

 which are of considerable interest in that they differ in a marked 

 degree from those to be observed in the other types to be discussed. 



In such a form as Erinocystis volborthi, Plate 2, figure 3, a marked 

 asymmetry is to be noted. One side of the theca is decidedly flat- 

 tened. The lower flattened side of the theca is well shown in figure 

 8, Plate 2, which illustrates a closely allied type, Ecliinoencrinus 

 angulosus. The anal opening has been carried outward and back- 

 ward to a marked degree and is situated at the end of an extraor- 

 dinary projection of the theca formed by the hypertrophy of certain 

 of the calycal plates. This lateral backward extension of the thecal 

 plates is especially to be noted in the case of Erinocystis angulata. 

 The column, as may be seen in Plate 2, figures 3, 8, is of the general 

 type characteristic of the group as a whole, 



Cheirocrinus. — In the genus Cheirocrinus we find imperfectly 

 developed the same general modification to bring about a flat- 

 tening of one surface of the animal that is to be noted in Pleuro- 

 cystis. Tliis flattening, instead of being acquired through the read- 

 justment and modification of plates already present in the theca, 

 is obtained through the excessive multiplication of circum-anal 

 plates. If this multiplication of plates be continued, it is obvious 

 that one side of the theca will in time largely be composed of such 



