282 PEOF. p. M. dtjncan's eevision op the 



close that the great differences in the structure of the apical 

 systems and in the interradial areas seem anomalous. There is 

 some variation in the size, position, and suturing of the basals in 

 one of the species with a disconnected bivium, but there have 

 been no specimens discovered in which any approach to a 

 blending of the two types of apical system has been seen. 



There is, moreover, a difficulty in deciding, if a division is to 

 take place, to which of the groups the name Pourtalesia should 

 belong, for Pourtalesia miranda, A. Agassiz, was the first 

 species described, and the nature of its apical system and the 

 development of its postero-lateral interradia do not appear to 

 coincide with P. Jeffreysi, "Wyv. Thoms., and its allies. 



In describing P. miranda, A. Agassiz wrote (Eevision, p. 345) 

 as follows : — " The posterior pair of ambulacra extend on both the 

 sides of an elongated plastron to the base of the snout-like pro- 

 longation, where they curve sharply upwards, and run close on 

 the abactinal part of the test to the abactinal system situated 

 almost at the summit of the nearly vertical anterior extremity, 

 along a well-marked wedge-shaped ridge extending from the 

 apical system into the rostrum protecting the anus." Again, 

 on p. 346, " The abactinal system, consisting of four large genital 

 openings placed close together, with the madreporic body toler- 

 ably well defined in the centre, is situated at the origin of the 

 anterior groove." 



On plate xviii., Revision of the Echini, fig. 1, a side view 

 of P. miranda shows the postero-lateral ambulacrum of the left 

 side, passing forwards abactinally to the very front of the test. 

 It is also evident in fig. 2 that the postero-lateral ambulacra 

 pass from the peristome on either side of the sternum far back, 

 and that each is continuous. In fig. 3 the apical system is 

 shown to be very excentric in front, and the postero-lateral 

 ambulacra reach it ; there is no junction of interradial plates 

 between the apical system and the bivium. Fig. 9 on the same 

 plate shows most distinctly that the apical system is compact, 

 and that the antero-lateral ambulacra do not terminate in radials 

 which separate the anterior and posterior basals, as in Echi- 

 nocorys. 



But in the classification P. miranda was placed by A. Agassiz 

 amongst the Ananchytidse (pp. cit. p. 344) ; and in his definition 

 of that group he states, " the apical system more or less elongate, 

 but not disconnected." The difficulty of associating the forms 



