148 Proceedings of the Biological Society of Washington. 



Kentucky. The other two species are from widely separated 

 localities and are found in earlier deposits. One comes from 

 the Black River Rhinidictya beds of Minneapolis, Minnesota, 

 and the other from the Chambersburg limestone near Chambers- 

 burg, Pennsylvania. 



The Minneapolis species has a very thin and fragile test, is 

 of smaller size, and has a surface covered bj" small granules. 

 The Chambersburg species is notable in having heavier plates 

 than Amecystis laevis and in the possession of well defined linear 

 surface sculpture, though pectinirhombs or porerhombs are 

 not developed. 



The absence of pectinii^hombs or even porerhombs in Ame- 

 cystis opens an interesting field of speculation as regards Cystid 

 evolution and systematic classification. It is scarcely con- 

 ceivable that Amecystis is a homoplastic derivative of a totallj'' 

 distinct genetic line from Plcurocystis. Owing to the essential 

 structural identity of Amecystis and Plcurocystis other 

 than in the possession of pectinirhombs we can scarcely go 

 further than postulate a common rhombless ancestor for both. 

 Indeed it is possible that forms referable to Amecystis were 

 ancestral to Plcurocystis. The age relations of the two genera 

 point to such a possible relationship. Although Amecystis 

 laevis (Raymond) ranges on into the Trenton, the other species 

 are of Black River age, and it will probably be found 

 that the genus had its greatest development in Chazj^an and 

 Black River times, whereas Pleurocystis is typical of the 

 Trenton. 



The more or less abrupt acquisition of porerhombs and even 

 the highly specialized pectinirhombs by genetic lines in which 

 stereom-folds are poorly developed or absent make the tran- 

 sition from the Amphoridea to the Rhombifera a simple 

 one. However, the assignment of Plcurocystis to the 

 Anomalocystidae as made by Haeckel is still un wan-anted. 

 The facts observed do suggest that the order Aporita 

 is unnecessary, and that the contained forms might well be 

 referred to the Rhombifera as now defined. 



