MESOZOIC FAUNAS 183 



towards " Heart-Urchin " characters than even the latest 

 Holectypoids, were represented in the Lias by Galero- 

 pygus, which, like Plesiechinus^ is abundant in the 

 Inferior Oolite. At that horizon it was joined by 

 Nucleolites, Clypeus and Pygurus. Gigantic tests of 

 Clypeus sinuatus give the name to the " Clypeus-gnt" 

 of Bathonian age, in the Coteswolds. The species 

 is historically interesting, having been recorded, and 

 credited with sundry virtues, in the seventeenth century. 

 Nucleolites (PL xiii. fig. 5), one of the simpler genera 

 of its order, is not uncommon in the Cornbrash, and 

 often swarms in the Corallian. It can boast of a single 

 surviving species. Pygurus lingered to the Lower 

 Cretaceous, but is rarely common in this country. 

 The Collyritidae, a peculiar group of small Echinoids 

 (probably an aberrant offshoot from the Nucleolitoid 

 stock), are represented by abundant species of Pygo- 

 rhytis in the Inferior Oolite. Hyboclypus, from the same 

 stage, was perhaps an incompletely specialized member 

 of the family. Spatangoida appeared at the beginning 

 of the Cretaceous period, and at once attained great 

 prominence. The imperfectly cordiform Echino- 

 corythidae are particularly characteristic of the period, 

 but persist in deep water at the present day. Holaster 

 supplies two zonal indices to the Chalk, and com- 

 prises many other common Upper Cretaceous species. 

 EchinocorySy an essentially Upper Chalk genus, can be 

 found at all horizons in that stage, and gives valuable 

 stratigraphical evidence by its protean changes of form. 

 The small Offaster is characteristic of high zones. True 

 Heart-Urchins (Spatangidae) are even more abundant. 

 Hemiaster is not uncommon in the Gault and Grey 

 Chalk, but its few British species give little indication of 

 the overwhelming vigour of differentiation shown else- 

 where by Cretaceous members of the stock. Epiaster 



