1896. ] GENUS OF FOSSIL ECHINODERMS, 1003 
The characters of Lysechinus which are unmistakable are the 
large mouth and apical system, and the small ambulacra occurring 
in grooves on the oral aspect of the test. These leave no doubt 
that Lysechinus is most nearly allied to Tiarechinus, and must be 
included as a second genus of Plesiocidaroida. It differs, however, 
from Tiarechinus in several important characters, of which the 
most remarkable are the limitation of the ambulacra to grooves on 
the oral half of the test, and the greater number of plates in the 
interambulacra. These differences are so important that it seems 
inadvisable to keep both genera in the same family, especially as 
neither genus appears to be on the direct line of descent of the 
other. 
I therefore propose to classify the order as follows :— 
ECHINOIDEA REGULARIA. 
Order PLESIOCIDAROIDA. 
Draenosts.—Echinoidea with a swall rigid test; peristome 
and periproct central and opposite. Periproct in the centre of an 
apical system of large plates, which constitute half of the whole 
test. The ambulacral areas are short and biserial. The inter- 
ambulacra begin with a single peristomal plate. There are no 
external gills. 
Family 1. TIARECHINIDS. 
Draenosis.—Plesiocidaroida with ambulacra with biserial pores. 
Fach interambulacrum consists of four plates, viz., a single peri- 
stomal plate, and three tall vertical plates in a horizontal row. 
Genus Trarecuinus, Neumayr, 1881. 
Species Tiarechinus princeps, Neumayr. 
Family 2. LysecHrnip2, 
Dracnosis.—Plesiocidaroida with ambulacra limited to grooves 
on lower surface of the test. Each interambulacrum begins with 
a single peristomal plate, succeeded by a row of two plates, and 
this by one or more containing three plates. 
Genus LYsnCHINUS, n. gen. 
Species Lysechinus tncongruens, N. sp. 
IV. Affinities of the PLESIOCIDAROIDA. 
After Loven’s skilful analysis of the test of Ziarechinus, and 
discovery of the constitution of the apical area, the genus became 
of great importance in Echinoderm morphology. The theory that 
the apical plates of echinids and the central dorsal plates of 
stellerids were homologous with the plates that form the calyx of 
