Prof. P. M. Duncan on the Salenidai. 71 



disc immediately before the anal opening ; this plate in some 

 genera is single, in others it is composed of from one to eight 

 elements " *. Found in strata of the Jurassic formation and 

 in those of the Cretaceous, the group is represented in the 

 Nummulitic and in the Australian Cainozoic deposits f 

 (probably Miocene). It still survives ; and two species of the 

 genus Salenia have been described from the result of deep-sea 

 dredging — one by Mr. A. Agassiz, under the name of Salenia 

 varispina, and another by Loven as Salenia GoesianaX- 

 Wright first explained the correct position of the anal plate, 

 by discovering the madreporic plate and appreciating its rela- 

 . tion to the longitudinal axis of the body of Salenia ; and 

 Loven's admirable researches on the antero-posterior axis 

 and the asymmetry of the ambulacra have confirmed the 

 truth elaborated by our great authority on British fossil 

 Echini. In his ' Revision of the Echini,'' pt. ii. p. 258, A. 

 Agassiz made the important generalization, which has been so 

 admirably worked out by Loven, that " the structure of the 

 abactinal system in young Echini explains most unexpectedly 

 the homology of the subanal§ plate of Salenidae." He stated 

 that the subanal plates have no special function, are not special 

 plates found in the group of Salenidge alone, but are simply 

 an embryonic feature retained in the adult ; and he concludes 

 that " this feature, which seemed so characteristic of a small 

 group of Echini, is one which alone has no primary systematic 

 value, so that we must, I think, hereafter consider the Sale- 

 nidge simply as a subfamily of the Cidaridse." Mr. Agassiz 

 gives an admirable description of the species of Salenia which 

 Count Pourtales dredged up off Florida in 275 fathoms ; and 

 as the buccal membrane was preserved he could notice that 

 his determination to abolish the Salenidge, Wright, and to 

 form a subfamily of the Cidaridge was not without its diffi- 

 culties ; for the serial continuation of the ambulacral tubes 

 does not take place through imbricated scales in the actinal 

 membrane. He instances, however, the Diadematidse as 

 presenting, in Diadema and Asthenosoma, similar differ- 

 ences. 



The general and minute structures, the nature and homo- 

 logies of the anal plates, and the classificatory position of the 

 Salenidae are thus full of interest ; and I propose to notice in 



* Wright, Palseont. Soc. Publ. vol. xxiv. p. 144. 



t Tate, Quart. Journ. Geol. Soc. vol. xxxiii. p. 256. 



X A. Agassiz, "Echini of the Eastern Coast of the United States," Illust. 

 Cat. Harvard Coll. no. vii. 1872, p. 2()1; Lov^n, "Etudes sur les Echi- 

 noides," Kongl. SvenskaVetenskaps-Akad. Handlingar, Bd. ii. no. 7, p. 27. 



§ = suranal of Wright. 



