224, Dr. T. Wright on the Cassidulidse of the Oolites. 



MontmoUini from the Neocomian stage. The single interambu- 

 lacrum is remarkable from having a central elevation on its dorsal 

 surface, two sinuous excavations commencing from the posterior 

 pair of ambulacra, and a central produced and deflected portion. 

 The vertex is excentral, in which the small apical disc is situated, 

 formed of four perforated ovarial plates and a single imperforate 

 plate, with the spongy madreporiform body occupying the centre 

 and five ocular plates the apices of the ambulacra. The base is 

 concave and much undulated, the ambulacra forming straight 

 valleys from the border to the mouth, and the intei ambulacra 

 convex eminences between them. Near the mouth their ter- 

 minal portions form five very tumid lobes around the margin of 

 that opening. The mouth is subpentagonal and placed diame- 

 trically opposite to the apical disc ; it is consequently excentral 

 and nearer the anterior border. The anal opening is not shown 

 in our specimen; according to Koch and Dunker, it is somewhat 

 ovate inclining to round, and is situated in the basal portion of 

 the produced and deflected single area. 



Affinities and differences. — This singular Urchin, in its elevated 

 anterior dorsal surface, very much resembles P. MontmoUini, 

 Ag., from the Neocomian stage of Switzerland, and P. irilobus 

 from the Craie chloritee of Maers; from the former it is di- 

 stinguished by the more angular outline of the posterior border, 

 from the latter it differs in having the central lobe less produced. 

 The form, in fact, is intermediate between them. 



Locality and stratigraphical range. — The specimen before me, 

 I was assured, was collected from the ferruginous beds of the 

 Inferior Oolite near Yeovil, and the lithological character of the 

 matrix supports the statement. It has been found by the officers 

 of the Geological Survey in the Coral Rag of Abbotsbury, Dorset- 

 shire, a fine specimen of which is in the Mus. of Pract. Geol. 

 Its foreign distribution is the Coral Rag de Jonnerre (Yonne) and 

 of Waltersberg (Hanover). 



History. — First figured and described by Koch and Dunker 

 in their monograph on the ' Norddeutschen Oolithgebilde/ 

 afterwards identified by Prof. Forbes in the collection made by 

 the Geological Survey in Dorsetshire, and now described as a 

 British Urchin for the first time. 



P y gurus pcntagonalis, Wright. PL VIII. fig. 3 a-e. 

 Syn. Echinanthite8 orbicularis, Young and Bird, Geol. York. Coast, 



pi. 6. fig. 5. 

 Clypeaster pentayonalis, Phillips, Geol. of Yorksh. tab. 4. fig. 24. 



Test with an ovoidal or subpentagonal circumference, much de- 

 pressed on the dorsal surface ; vertex nearly central, in which 

 the apical disc is situated; ambulacral area petaloid, broad, 



