ARGOPATAGUS AND GENICOPATAGUS. 



173 



87, figs. 1-3) the left posterior interambulacral plates attain a great width. 

 In Phrissocystis (Pis. 88, figs. 1, 2; 89, fig. 2), Argopatagus (PI. 91, fig. i), 

 and specially in Genicopatagus (PI. 91, figs. ^, 5) the interambulacral and 

 the ambulacral plates are of more uniform size, the latter increase very 

 gradually in size towards the ambitus -, in Argopatagus alone the posterior 

 ambulacral plates on the actinal side increase rapidly at the ambitus towards 

 the actinostome (PI. 91, fig. 2). In Argopatagus, as will be seen on Plate 

 91, fig. 2, the second plates of the posterior zone of the posterior lateral 

 ambulacra almost separate the labium from the sternum, as in Plexechinus; 

 this is an indication of the affinities of the genus to the Pourtalesiae. 



As regards the abactinal system, that of Palaeotropus, Fig.s. 245, 247 (PI. 

 87, fig. 6), of Argopatagus (PI. 91, fig. s), Fig. 252, of Phrissocystis (PJ. 89, 

 fig. S), Fig. 276, and of Homolampas, Fig. 253 (PI. 63, fig. J) all belong to 



48 mm. ? 



Fig. 252. Akgopatagus vitreus. 



32 mm. 



Fig. 253. Homolampas hastata. 



the same type ; the genital plates are ankylosed in the midst of the madre- 

 poric body, which covers the greater part of the central plate formed by the 

 genitals. While in the abactinal system of Genicopatagus (PI. 91, hg. 6) 

 three of the genital plates are well developed ; the fourth, carrying no genital 

 opening, is ankylosed with the madreporite plate, it extends from the right 

 anterior interambulacrum to the odd interambulacrum and separates the left 

 genital plates from the right posterior plate, Fig. 254, — a type of abactinal 

 system belonging to Schizaster (Figs. 297, 301), far more recent than that 

 of the other genera just named, though as a whole the other features of the 

 test of Genicopatagus are more antique. The Palaeotropus type of abactinal 

 system is also found in Agassizia Fig. 287 (PI. 108, fig. 7), perhaps in Spatago- 

 desma (PI. 106, fig. 2), Fig. 289, and in Neolampas (PI. 64, figs. 7, S) ; it may to 

 a certain extent be called Echinolampid, occurring as it does in Echinolampas 

 and Conolampas (PI. 65, figs. 4, 7). 



