FROM THE INFERIOR OOLITE. 295 



The whole of the plates on the dorsal surface are covered with numerous, minute, 

 regular, and nearly equal-sized, perforated tubercles (fig. \f,(j), which are raised on bosses 

 with crenulated summits, and surrounded by depressed, ring-like areolas ; on one plate 

 near the border I have counted as many as one hundred tubercles ; the inter-tubercular 

 surface of the plates is, besides, strewed with microscopic granules, which form circles 

 around the areolas, and fill up the intervening spaces (fig. 1/) ; the tubercles at the base 

 are much larger and better developed than those on the dorsal surface ; at the border and 

 outer third they are very numerous and set close together; the areolas here are 

 diamond-shaped, or hexagonal, and are more excavated, a single row of granules only 

 separates the areolas from each other ; towards the mouth they are larger, and not so 

 regularly arranged, having several rows of granules between them (fig. 1 e). 



Affinities and differences. — The adult forms of H. ac/ariciformis, Forbes, differ so 

 widely from its other Oolitic congeners, that this urchin cannot be mistaken for either of 

 them ; its sub-orbicular shape and depressed dorsal surface distinguish it from II. gih- 

 berulus, Agassiz, and H. ovalis, Wright ; the central position of the apical disc, and the 

 absence of any elongation of the single inter-ambulacrum, are diagnostic distinctions 

 between it and H. caudatus, Wright. From the Pygasters, with which some of the 

 species have been erroneously grouped, it is distinguished by the microscopic character 

 and greater number of its tubercles ; the deep anal valley, with vertical walls, in which 

 the vent opens, the small sub-central mouth opening, and narrow apical disc, form a 

 group of generic characters by which the Hyhodypi are distinctly separated from the 

 Pi/oasters ; whereas the narrow poriferous zones, complete and continuous from the mouth 

 to the apical disc, without any petaloidal expansion on the dorsal surface, distinguish 

 Hyhoclypus from Clypeus, Echinohrissus, and Pyyuncs. 



Locality and Stratiyraphical position. — This urchin is very abundant in the lower 

 ferruginous beds of the Inferior Oolite, "the Pea Grit" of Leckhampton, Crickley, 

 Cooper's, Cleeve, and Sudely Hills, and at Camlong Down, near Uley Bury, in Glouces- 

 tershire. It was collected from the Inferior Oolite by the Geological Surveyors between 

 Wayford and Seaborough in Dorsetshire, where it was accompanied by Collyriies rinyens, 

 Agassiz, and Ilolectypus liemisphcBncus, Desor. I have found two specimens in the Great 

 Oolite of Minchinhampton, Gloucestershire, along with Purpurina Ilorrisii, P. nodulata^ 

 Young, and other species of Mollusca characteristic of that formation. 



History. — This species was first described in my Memoir on the Cassidulid^ of the 

 Oohtes, and afterwards figured by Professor Forbes in the IV Decade of the ' Memoirs of 

 the Geological Survey.' In consequence of the opening for the apical disc having a 

 denticulated border, M. Cotteau has proposed for it the new genus Galeopyyus ; but as I 



