EEPORT ON THE EADTOLARIA. 



5(33 



a true phylogenetic relation. But it is remarkable that we do not find further forms of 

 development in this family, by multiplication either of the arm-chaml)ers (further 

 growth in the three perradii) or of the concentric chambered rings (in the periphery of 

 the disk margin). 



The central capsule of the Pylodiscida is constantly flat, discoidal, and enclosed 

 between the two sieve -plates of the surface. Its form is either circular or triangular. 



I. Subfamily 

 Ti'iopylida. 

 Three gates between three 

 simple arm-chambers. 



Synopsis of the Genera of the Pylodiscida. 

 Three gates open, without a barring equatorial girdle, 



Three _ gates barred by a j ^^^te-faces simple, . 



latticed equatorial girdle. 



Gate-faces latticed, 



II. Subfamily 



Hexapylida. 

 Six gates between three double 

 arm-chambers (three inner 

 and three outer) ; no cham- 

 bered marginal girdle. 



III. SubfamQy 



Discopylida. 



Six gates between three 

 double arm-chambers. 



Three outer gates open, without a Ixirring equatorial girdle, 



f Both faces of the outer 

 Three outer gates barred by j gates simple, 

 a latticed (second) equa- -' 



I 



torial girdle. 



In the equatorial plane on 

 the margiu of the Pylo- 

 iJiscus-shell is a cham- 

 bered equatorial girdle. 



Both faces of the outer 

 gates latticed, . 



No p)eculiar osculum on the 

 margin of the disk. 



One peculiar osculum (with 

 a corona of spines) on the 

 margin of the disk, 



24.5. Triolcna. 



246. Triopiile. 



247. Triodiscus. 



248. Pi/Jolena. 



249. Hexapijle. 

 2.50. Pylodiscus. 



251. Discomnhim. 



252. Discopyle. 



Subfamily 1. Triopylida, Haeckel. 



Definition. — P ylo disci da with a simple, spherical or lenticular, central chamber, 

 surrounded by three simple arm -chambers, which are separated by three notches or gates. 



• Genus 245. Triolena,^ n. gen. 



Definition. — P y 1 o d i s c i d a with a simple, spherical or lenticular, central chamber, 

 surrounded by three simj^le arm-chamliers. Notches between the three arms open. 



The genus Triolena is the most simple form of all Pylodiscida, and must be 

 regarded as their common ancestral form, from an ontogenetic as well as a phylogenetic 

 point of view. The small shell is composed of a simple, spherical or lenticular, latticed, 

 central chamber, and of three simple, surrounding equal arms, which are also simple 

 latticed chambers, lie in the equatorial plane, and are separated by three equal angles 

 or open gates. 



' Tn'ofeTOts Shell with three arms ; tj/*, uT-iun 



