83 



CLASSIFICATION OF THE 



is admitted to be a true Diadema. The valuable memoir of Herr W. Peters^ has 

 removed some of the difficulties that surrounded this subject, and his grouping of the 

 living Diademas makes an important step towards a natural classification of one section of 

 this Family. Although the present state of our scientific knowledge of the Diadeniadts 

 may be considered as transitional rather than positive, still we possess enough to justify 

 the separation of fossil Diademas from existing genera, as proposed by M. Desor." 



The DiADEMADiE, in fact, appear to consist of two types ; one of these, with a few rare 

 exceptions, appertains to the present epoch, the other existed diu'ing the deposition of the 

 Secondary and Tertiary rocks. The living forms arc in general large, depressed Urchins, with 

 thin shells, having the tubercles and pores variously arranged in the different genera. They 

 have, in general, very long, slender, tubular spines, and the surface of the stem is covered 

 with oblique annulations of small imbricated scales. The fossil species, on the contrary, 

 are smaller Urchins, with a thicker test ; having the tubercles and pores variously disposed 

 in different genera; the spines rarely attain the length of the diameter of the test; 

 they are in general solid, cylindrical, sometimes flattened or awl-shaped, and their surface 

 is covered with fine longitudinal lines. I propose to include the following genera in this 

 natural family. 



A Table shoimng the Classification of the Diademada. 



Section a. 

 / Spines very long, slender, tubular, 



DIADEMAD^ / 



\ 



covered with oblique annulations 

 of imbricated scales. Living in 

 tropical seas. 

 A few annulated tubular .spines are 

 found iu the Upper Chalk and in 

 the Coralline Crag. 



Section b. 



Spines short, slender, solid ; surface \ 

 covered with fine longitudinal 

 lines. 



E.xtinct ; found in the Oolitic, Cre- 

 taceous, and Tertiary Rocks. 



DiAUEMA, Gray. 

 Savignya, Desor. 

 AsTROPYGA, Gray. 

 EcHiNOTiiEix, Peters. 



PSEDDODIADEMA, Desor. 



CyPHOSOMA, Agassi:. 

 Hemipedina, Wriyht. 



Pedina, Ayassiz. 

 EciiiKOPsis, Ayassiz. 



My learned friend M. Cotteau,'' in his classical work on the Echinidae of France, has 

 lately proposed an extended classification of the family Diademad^, a resume oi which I 



1 ' Ueber die Gruppe der Diademen, Konigl. Akademie der Wissenschaften,' Berlin Aug., 1853. 



- ' Synopsis des Echinides Fossiles.' 



^ ' Paleontologie Eranoaise, Terrain Cre'tac^,' tom. vii, p. 3"1. 



