1896.] GENUS OJf EOSSIIi EOHINODEBIIS. 1003 



riie 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 LijsecJiinus is most nearly allied to Tiarechinus, and must be 

 included as a second genus of Plesiocidaroida. It differs, however, 

 from Tiarechinus in several impoitant 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 : — 



ECHINOIDBA EBQULAEIA. 



Order PLESIOCIDAEOIDA. 



Diagnosis. — Echinoidea with a small 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. Tiaeechinid-s. 



Diagnosis. — Plesiocidaroida with ambulacra with biserial pores. 

 Each interamhulacrum consists of four plates, viz., a single peri- 

 stomal plate, and three tall vertical plates in a horizontal row. 



Genus Tiaeeciiinus, Neumayr, 1881. 



Species Tiarechinus princejis, Neumayr. 



Family 2. LYSEOHiNiniE. 



Diagnosis. — 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 Lysechinus, n. gen. 



Species Lysechinus incongruens, n. sp. 



IV. Affinities of the Plesiooidaeoida. 



After Loven's skilful analysis of the test of Tiarechinus, 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 



