252 ASTEROIDEA AND OPHIUROIDEA. 



not detain us here, as they are almost wholly referable to 

 living genera, though the species are distinct. In the Trias 

 (Muschelkalk), we have the genus Pleuraster, doubtfully 

 separable from Astropeden. In the Jurassic period we have 

 the earliest representatives of the living genera Uraster, 

 Solaster, Luidia, Astrogonium, arid Goniaster. The existing 

 genus Astropeden (fig. 144), easily recognised by the great 

 spine-bearing marginal plates, is largely represented ; while 

 the extinct Tropidaster forms a link between this and Uras- 

 ter ; and Plumaster, also extinct, is a near ally of the recent 

 Luidia. 



In the Cretaceous rocks almost all the known forms 

 belong to existing genera (such as Goniaster, Stellaster, 

 Astrogonium, Palmipes, and Oreaster) ; while in the Tertiary 

 deposits we meet only with the generic types of the present 

 day. 



AGELACRINIM. 



We may provisionally consider here a most extraordinary 

 group of Palaeozoic Echinoderms, the precise affinities of 

 which are at present wholly uncertain, though they appear 

 to be in some respects intermediate between the Asteroidea 

 and the Cystoidea. The singular forms in question have 

 been grouped together by Mr Billings under the name of 

 Edrioasteridm, but are better entitled Agelacrinidce ; and they 

 fall under the two related generic types Agelacrinus and 

 Edrioaster. 



In the genus Agelacrinus or Agelacrinites (including Hemi- 

 cystites of Hall) the body (fig. 145, A) is in the form of a 

 depressed or convex disc, attached by its base to some 

 foreign body. The upper surface of the disc (which is 

 really the ventral surface) is covered with numerous small 

 calcareous plates, which may or may not overlap in an 

 imbricating manner, and exhibits five curved " arms," which 

 radiate from the centre. The rows of plates forming 

 the arms are so disposed, in some instances at any rate, 

 as to leave between them distinct "pores," penetrating 

 the thickness of the test; so that the arms clearly corre- 



