40 PALEONTOLOGY. 



Gassidididcu commence in the oolites, with Pygaster (fig. 7, i) 

 and Holectypus, and abound in the cretaceons system. Gale- 

 rites (fig. 7, 3), Discoidea, Pyrina, and Cassidulus are pecu- 

 liar to the chalk. The Glypeastridw are represented in the 

 oolites by numerous species of Echinolampas and Nucleolites 

 (or Glypeus) ; the latter genus attains a large size. The sub- 

 genus G atopy gus (fig. 8,. 8) is peculiar to the cretaceous series. 

 Gonoclypeus occurs in the chalk and tertiaries. Glypeaster 

 flourished most in the miocene age ; many large species are 

 found in the south of Europe, Madeira, and the West Indies. 

 Numerous genera, remarkable for their flattened form, and 

 popularly known as " cake-urchins," are peculiar to the ter- 

 tiaries and existing seas. Lenita and Scutellina are eocene ; 

 Scutella (fig. 7, 4) is miocene. Mellita and Echinarachnius 

 are both fossil and recent. The heart-shaped urchins (Sp>atan- 

 gidce), are only remotely represented in the oolites by Disaster 

 (fig. 8, 6) ; they are numerous in the chalk, to which Micr aster, 

 Epiaster, Hemipneustes (fig. 8, 7), Archiacia, Holaster, and 

 Ananchytes (fig. 7, 2), are peculiar. Toxaster is characteristic 

 of the lower neocomian. Hemiaster is cretaceous and tertiary. 

 Spatangus, Eiqmtagus, Brissus, Amphidohts, and Schizaster are 

 tertiary and recent forms. 



The shell of the Echinodermata has the same intimate 

 structure in all the orders and families, and in every part of 

 the skeleton, whether "test," or "spine," or "tooth." The 

 smallest plates resemble bits of perforated card-board, and the 

 largest and most solid are formed of a repetition of similar 

 laminae. In a few membranous structures, minute spicula, 

 curved, bi-hamate, or anchor-shaped, are met with. They are 

 always composed of carbonate of lime; but owing to their 

 porosity, fossil examples are commonly impregnated with 

 earth, or pyrites, or silica, and form bad subjects for micro- 

 scopic investigation. Without, however, losing their organic 

 structure, the fossil Echinoderms exhibit a cleavage like that 



