﻿23^ SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



difference is that whereas plates 10 and 16 in Pseudocrinites are 

 situated in the third and fourth rows, in Trimerocystis they have 

 passed one circle lower, being, respectively, in the second and third 

 rows. In tabular form the 4 rows of 19 thecal plates in these genera 

 are arranged as follows : 



Pseudocrinites 

 Trimerocystis 



Basal Circle 



Circle 2 Circle 3 



4-I-2-3 5-6-7-8-9 



4-I-2-3 10-5-6-7-8-9 



IO-II-I2-I4 

 16-II-12-I4 



Circle 



16-I7-1S-I3-I9-15 

 17-1S-I3-I9-15 



Madreporite and hydropore as in Pseudocrinites. 



Genotype, T. peculiaris n. sp. 



Comparisons. — The 3 long ambulacra readily distinguish Trimer- 

 ocystis from all other related genera, as they have either 2 or 4 simple, 

 or 4 variously branched ambulacra. 



In determining what 3 ambulacra of the 5 of a primitive cystid are 

 present in Trimerocystis, the writer has relied on the assumption that 

 R III is always absent in forms having 4 ambulacra, as Tetracystis, 

 Lepocrinites, etc. This view seems to be correct, since in all 4-armed 

 cystids described in this paper, there is no ray directly opposite the 

 anus. In Pseudocrinites Jaekel identifies the 2 ambulacra as R II and 

 R V ; Bather does the same, except that he names them "apparently 

 right posterior and left anterior." If this interpretation is correct, 

 then the third ambulacrum, or the one just to the right of the anal 

 area, in Trimerocystis should be R IV or the right anterior ray, thus 

 placing the anal area between R A" and R IV, which is of course 

 anomalous and highly improbable. It is certain that the ambulacrum 

 to the left of the anal area in Trimerocystis is the same as R II in 

 Pseudocrinites; also that the one passing anteriorly in the former 

 genus is equal to R V of the latter — an interpretation also made by 

 both Jaekel and Bather. This is further shown in the specimen of 

 'Trimerocystis in which the 2 corresponding ambulacra are con- 

 tinuous, and directly in line and opposite to each other as in Pseudo- 

 crinites. Not only so, but the madreporite. and pectinirhombs 1 and 

 5. and 12 and [8, are to the left of these ambulacra, while the anal 

 area and pectinirhombs 14 and 15 are to the right in both genera. 

 The third ambulacrum in Trimerocystis is also to the right. If this 

 third ambulacrum is not altogether anomalous, then it may be stated 

 thai the 2 ambulacra placed one on either side of the anus represent 

 the bivium or rays I and V. As the anterior ray does not hold 

 the central line opposite the bivium, one is seemingly forced t<» con- 

 clude that it represents R IV. With this interpretation the 2 ambu- 



