Dr. T. Wright on the CassiduluUe of the Oolites. 191 



widest, has attached to its surface a spongy prominent body ; 

 the posterior pair arc smaller, and both pairs are perforated ; the 

 single ovarial is the smallest and is imperforate; the ocular 

 plates are small pentagonal bodies, wedged between the angles 

 formed by the ovarials ; they are placed opposite the summits of 

 the ambulacra, and have the eyeholes pierced at their lowest 

 border. The anus is a large opening situated in the margin of 

 the single interambulacrum ; it is of a pyriform shape, having the 

 base directed towards the mouth, and the apex extending some 

 distance, about one-fourth part up the side ; the proportional size 

 of the opening varies in the suite of specimens before me; in all, 

 however, it is constantly marginal. 



Affinities and differences. — This species nearly resembles H. 

 depressus in its hemispherical form and general outline ; it is 

 readily distinguished from it however in being a little more 

 elongated, in having the anal opening marginal and the vertex 

 and apical disc slightly excentrical, but the marginality of the 

 anus is the most important diagnostic character. 



Locality and stratigraphical range. — This species was collected 

 at Shurdington Hill with H. depressus, where it is rare; it was 

 collected in abundance by Messrs. Bristow and Gapper* along 

 with Dysaster ringens and D. bicordatus from the sands of the 

 Inferior Oolite in Dorset and Somerset. M. Desorf found it in 

 Normandy in a particular bed of the Calcaire au Polypiers known 

 by the name of Caillasse, and M. Cotteau % obtained it from the 

 Inferior Oolite of Tour-du-Pre, associated with Dysaster ringens 

 and Diadema dejjressum. 



History. — First figured and described by M. Desor in his 

 ' Monograph des Galerites/ and lately exquisitely figured in detail 

 and admirably described by Prof. Forbes in the 3rd Decade of 

 his ' Palaeontological Memoirs of the Geological Survey/ 



Genus Hyboclypus, Agassiz. 



Urchins having a circular, oblong, or subpentagonal circum- 

 ference, with the dorsal surface in general much depressed. The 

 ambulacral areae meet above at two points as in the genus Dys- 

 aster ; the single and anterior pair of ambulacra terminate at the 

 anterior border of the apical disc, the posterior pair at a short 

 distance from them. The anus is situated in a deep valley ex- 

 tending from the vertex to the posterior border of the test. The, 

 is much undulated; the mouth is excentrical, placed nearer 

 the anterior border; it is of a pentagonal form and has no mar- 

 ginal folds. The surface of the test is covered with small close- 



* Memoirs of Geol. Survey. Decade iii. 

 t Monogr. des Gnlmtes, p. 71- 



* Etudes des Kchinults Fossiles. p. 46. 



