286 PYGASTER. 



all the disc plates is covered with numerous small granules ; and still smaller granules are 

 scattered over the surface of the madreporiform body (fig. 2 e). 



Affinities and differences. — This species very much resembles Pygaster semisulcatus, 

 Phil., and was for a long time mistaken for that species ; Professor Phillips having stated 

 that P. semisulcatus was common to the Great (?) Oolite of Whitwell and the Coralline 

 Oolite of Malton ; this, however, I have proved, by an examination of the specimens, to be 

 an error, the Malton Piasters being quite distinct from the Whitwell species. 



In P. umbrella the tubercles on the upper surface are always smaller, and not very 

 regularly arranged in vertical and horizontal rows ; the bosses are less elevated, the 

 areolas less defined, and the intervening granules smaller and fewer in number than in 

 P. semisulcatus. The discal opening is smaller; the anal opening is longer, more 

 pyramidal, and seldom symmetrical, occupying sometimes two thirds of the area ; the basal 

 tubercles of both species are nearly alike in size ; in P. umbrella, however, there is a much 

 greater disproportion between the size of the tubercles on the upper and under surfaces of 

 the test than exists in P. semisulcatus. 



P. umbrella is easily distinguished from P. Morrisii by the size of the tubercles in the 

 latter, its more decided pentagonal form, larger ambulacra, and tumid sides. 



P. dilatatus, as stated by MM. Agassiz and Desor, " Se distingue du P. umbrella par 

 son bord plus tranchant et sa forme plus dilatee." The shape of the anal opening is 

 different, and it wants the pyramidal figure so characteristic of P. umbrella ; it belongs 

 likewise to a different stratigraphical horizon, having been collected from the " Portlandien 

 du Jura Soleurois, carriere de Greifel (vallee de la Birse) ;" whilst P. umbrella is found 

 only in the Coral Rag. 



Locality and Stratigraphical position. — I have collected this urchin from the lower 

 calcareous grit at Headington, near Oxford; from the Coralline Oolite of Malton, 

 Yorkshire ; from the Coral Rag near Farringdon, Berks ; and from the Coral Rag of 

 Lyneham and Calne, Wilts ; in the two latter localities it was associated with Cidaris 

 florigemma, Pseudodiadema versipora, P. mamillanum, and Ec/iinobrissus scutatus — all true 

 Corallian forms. 



On the Continent it has been collected by M. Cotteau from the " Calcareo-siliceuses des 

 environs de Druyes, a Chatel-Censoir, et a, Montillot (Yonne);" by M. Buvignier from the 

 Coral Rag environs of Saint Mihiel (Meuse) ; and M. Saemann, of Paris, kindly sent me a 

 specimen which was obtained from the Coral Rag of Commercey (Meuse). 



