CYSTECHINUS. 107 



There is considerable variation in U. Naresianus in the manner in which 

 the bivium is separated from the triviuin by the plates intercalated be- 

 tween the posterior genitals and the anterior lateral ambulacra (Fig. 227). 

 When the right anterior genital is greatly developed it conges in contact with 

 the posterior genitals so that the belt of intercalated interambulacral plates 

 does not extend across the abactinal system. Similar variation in the 

 arrangement of the abactinal plates (Fig. 219) has been observed in 

 Pilematechimis Rathhuni (Figs. 238, 239). 



Station 3431, off Altata, Gulf of California, in 995 fathoms. Lat. 23" 

 59' N. ; Long. 108° 40' W. Bottom temperature 37\ Lt. br. m. glob. 



Ctsteominus a. Ag. 



Gregory has described a species of Cystechinus^ (C. crassits) from the 

 Radiolarian marls of Barbados. " The specimen is unfortunately in a very 

 imperfect state of preservation, as the actinal side is not shown, and the 

 structure of the apical system can only be inferred. Even on the abactinal 

 side many of the plates have been lost." Mr. Gregory further states that 

 none of the "Challenger" collections of Cystechinus in the Natural History 

 Museum are more perfect, except a couple of very young specimens of C. 

 W/jrillH, and that the specimen is in better preservation than the collection 

 of scattered plates that forms the type of C. clf/jjeafus. The great thickness 

 of the plates of C. crassiis and the resemblance to those of the Palaechinidea 

 would seem to preclude the association of this species with Cystechinus, 

 especially as we know nothing of its shape, of its actinal face, or abactinal 

 system.^ Only one of the species of Cystechinus described from the " Chal- 

 lenger" collection {C. vesica A. Ag.) possesses a flexible test,^ and not others 

 as is suggested by Gregory ; the others, though having a thin test, were rigid. 



It is hardly correct to say that none of the "Challenger" specimei;-'. of 

 Cystechinus were in a better condition than the fragment figured by Gregory, 

 since it has been possible to make such drawings of C. vesica, C. WyvilJii, and 

 C. clypeatus* as we have given in the "Challenger" Echinoidea. The com- 



prorainent lower lip and an elliptical crescent-shaped actinostome. It has also a well developed 

 subanal fascicle, as in some specimens of Urechinus Naresianus ("Challenger" Echinoidea, PI. 

 XXX a, figs. 10, 10 a, 12, 13). 



1 Quart. J. Geol. Soc. London, 1889, Vol. XLV. p. 040. 



" It is true that the plates of the test of deep sea species vary somewhat, but not to the extent 

 assumed by (Iregory in uniting C. crassiis to Cystechinus. 



* A. Agassiz, "Challenger" Echinoidea. p. 1")1. 



* "Challenger" Echinoidea, Vh. XXIX, XXIX », XXIX b, XXXV, XXXV », figs. 5-8. 



