Q-EWEEA ATSTD GEOTJPS OF THE ECHINOIDEA. 285 



or clubbed. Pedicellariee, some trifid, witb circular tops with 

 milled edges. 



Recent. Atlantic, Pacific, and Antarctic Oceans; 345 to 2900fms. 



This diagnosis will embrace the following species -.—Fourtalesia 

 Jeffreysi, Wjy. Thorns., P. laguncula, A. Agassiz, P. Msjpida, 

 A. Agassiz, P. pJiiale, "Wyv. Thoms., P. ceratopyga, A. Agassiz, 

 P. carinata, A. Agassiz. 



Of these the last two have the sternum connected and the 

 interradia 1 and 4 do not unite at the median line actinally, but 

 only abactinally. 



The diagnosis will not admit Fourtalesia miranda or P. rosea, 

 A. Agassiz, which have a compact apical system and the postero- 

 lateral interradia separated dorsally. These species should belong 

 to another genus ; but as the morphology is as yet unsatisfactory, 

 it is best to place them provisionally in a division of Fourtalesia. 



Although the diagnosis of Fourtalesia just given, will suffice to 

 distinguish it from any other genus, there are some points in the 

 morphology of some of its species which should be recorded, for 

 they are of especial value in showing the aberrant character of 

 the group, how it may be associated, in a family, with other genera, 

 and how slightly it is allied, structurally, to any ancient forms. 



Loven has shown that the inversion of the anterior part of the 

 test not only afiects the anterior ambulacrum, but also the peri- 

 stome and the first plate of the odd interradium. Each side of 

 the peristome, which is narrow and elongate from below upwards, 

 is formed by a narrow and long plate of zone a of the antero-lateral 

 interradium. The first plate of zone 5 may enter, so that there is 

 not a " single " peristomial plate ; the upper margin is formed of 

 the anterior ambulacral plates. The marginal plates of the lateral 

 ambulacra and the small lip of the posterior, odd, interradium 

 are raised vertically to the actiual plane of the test, so that the 

 mouth is entirely within the inversion. In P. Jeffreysi the great 

 height of the first plates of the zones («) of the antero-lateral 

 interradia prevents the antero-lateral ambulacra from entering 

 the peristomial margin. 



The interradia 1 and 4 in most of the species are large and 

 form a belt round the test, and it is evident, from Loven' s work, 

 that they do not enter into the composition of the peristome, and 

 indeed are remotely posterior to it. 



The large plates of these interradia are very striking, and it 

 has been explained by Loven, op. cit. p. 14, that the heteronomy 



