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ECHINOCONIDvE. 



"Fig. 5. All Echinodiscus, one half of the circumference marked with unequal indenta- 

 tions. The other half furnished with two pervious apertures. 

 Fig. 6. The base, with mouth and anus. 



Fig. 7. The Echinodiscus maximus, with margin entire. The anal aperture placed 

 on the margin itself. From the Kleinian Museum. 



Fig. 8. The base, with the oral aperture in the centre. 



A new genus may not inaptly be constituted as the eighth in order in my 

 ' System,' from this last species, since it differs as to the rule of the position of the anal 

 aperture, and in the absence of the representation of the flower. But since only this 

 single species has been known hitherto, I have preferred adding it to the Echinodisci, 

 until perchance some other specimen shall have been discovered." 



[I must refer all interested in the study of this group to Professor L. Agassiz's 

 admirable Monograph ' Des Scutelles,' with magnificent plates of living and fossil forms. 

 T. W.] 



VI. Family — Echinoconid^e, Wright, 1854. 



When I proposed the establishment of this Family I defined it as a natural group of 

 fossil Echinoidea having a thin, circular, or slightly pentagonal test ; the upper surface 

 in most of the forms being very much elevated or conoidal, in others it is more or less 

 depressed. 



The ambulacral areas are narrow and the inter- ambulacral wide ; the plates of both 

 are covered with numerous, small, perforated tubercles, raised on bosses with crenulated 

 summits. They are sometimes scattered over the plate, but are frequently arranged in 

 regular longitudinal rows. They are always larger at the base than on the sides and 

 dorsum ; and the surface of the test is likewise covered with close-set microscopic 

 granules. 



The poriferous zones are narrow, and formed throughout of round unigeminal pores 

 about equal in diameter; they converge in a straight line from the apical disc to the 

 peristome, around which aperture they have sometimes a bigeminal arrangement. 



The mouth-opening is inferior, central, sub-circular, and armed with five pairs of jaws ; 

 the peristome is more or less decagonal and divided by notches into ten lobes, well 

 marked in Pygaster and Holectypus, but feebly in Discoidea and Echinoconus. 



The vent is variable in position ; it is situated at the upper surface in Pygaster, at the 

 border in EcMnoconus, at the base in Discoidea ; and this -aperture is oval, pyriform, or 

 oblique in different genera. 



The apical disc occupies the summit of the upper surface, and is composed of five ovarial 

 and five ocular plates ; the madreporiform body is very large, extending from the right 

 antero-lateral ovarial into the centre of the disc. 



