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Show 1896.] GENUS or FOSSIL E C H I N O D E R M S . 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 Tlarechinus, and must be included as a second genus of Plesiocidaroida. It differs, however, from Tlarechinus in several important characters, of which the most remarkable are the limitation of the ambulacra to grooves on tbe oral half of the test, and the greater number of plates in tbe 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 KEGULARIA. Order PLESIOCIDAROIDA. 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 interambulacra begin with a single peristomal plate. There are no external gills. Family 1. TIARECHINID^E. DIAGNOSIS.-Plesiocidaroida with ambulacra with biserial pores. Each interambulacrum consists of four plates, viz., a single peristomal plate, and three tall vertical plates in a horizontal row. Genus TIARECHINUS, Neumayr, 1881. Species Tiarechinus princeps, Neumayr. Pamily 2. LYSECHINIDJS. 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 L Y S E C H I N U S , n. gen. Species Lysechinus Incongruens, n. sp. IV. Affinities of the PLESIOCIDAROIDA. After Loven's skilful analysis of tbe test of Tlarechinus, 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 |