108 



DIADEMADtE. 



this family. Although the present state of our scientific knowledge of the Diademadce 

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

 separation of the fossil Diadems from the existing genera, as proposed by M. Desor.* 



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

 exceptions, appertains to the present epoch ; the other existed during the deposition of the 

 secondary and tertiary rocks. The living forms are in general large, depressed urchins, 

 with a thin shell, having the tubercles and pores variously arranged in the different genera. 

 They have, in general, very long, slender, tubular spines, the surface of which is covered 

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

 are smaller urchins, having a thicker test; the tubercles and pores are variously disposed in 

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

 are solid, in general 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 showing the classification of the Diademadce. 



Section a. 



Spines very long, slender, tubular, covered with 

 oblique annulations of imbricated scales. Living in 

 tropical seas. 



A few annulated tubular spines are found in the 

 1)1 VDEMADiE. Upper Chalk and in the Coralline Crag. 



Section B. 



Spines short, slender, solid ; surface covered with 

 fine longitudinal lines. 



Extinct in the Oolitic, Cretaceous, and Tertiary 

 rocks. 



Diadema, Gray. 

 Savignya, Desor. 

 Astropyga, Gray. 

 Echinothrix, Peters. 



Pseudodiadema, Desor. 

 Cyphosoma, Agassiz. 

 Hemipedina, Wright. 

 Pedina, Agassiz. 

 Echinopsis, Agassiz. 



Pseudodiadema,! Desor. 1854. 



This genus is composed of small urchins, with a moderately thick test, which rarely 

 attains two inches in diameter ; the ambnlacral areas are in general one third, or even one 

 half the width of the inter-ambulacra ; the primary tubercles of both areas are all perforated, 

 and nearly of the same size; their bosses are small, with sharply crenulated summits. 



The ambulacral areas have two rows of tubercles ; the inter-ambulacral areas have two 

 rows only, or two rows of primary, and two or four shorter rows of small secondary 



' Synopsis des Eehinides Eossiles.' 



The specific name given by Lamarck to the fossil urchin described as Cidarites pseudodiadema. 



