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Genus— CLjmVS, Klein, 1734. 



This genus includes all the large discoidal m'chins, with petaloid ambulacra, in 

 which the vent opens at the upper surface into an anal valley ; the dorsal portions of the 

 poriferous zones are widely petaloidal ; the ambulacral summit is central, or excentral, 

 and when this is the case, the excentricity is always towards the posterior border ; the 

 elements of the apical disc are closely soldered together, there are two pairs of perforated 

 genital plates, one single imperforate plate, and five ocular plates, the orbits are in general 

 marginal and visible, and the spongy madreporiform body occupies the centre of the disc. 



The base is flat, concave, or undulated, in diflferent species ; the mouth-opening is 

 small and lodged in a depression, which is either central or sub-central ; the peristome is 

 surrounded by five oral lobes, formed by the terminations of the inter-ambulacra. 



This group differs so slightly in all its essential characters from the genus Echinobrissus, 

 that I formerly agreed with my late friend. Professor Forbes, to include all the species of 

 Clypeus in the Lamarckian genus Nucleolites. After a more detailed study of the compara- 

 tive anatomy of their tests, taking into consideration the magnitude and development of 

 the long wide petaloidal poriferous zones, and the relation these parts had to the internal 

 organs, I have determined to retain the genus. 



Like Echinobrissus, the genus Clypeus includes two types of structure ; in the first, 

 the anal valley extends from the apical disc to the posterior border, this section includes 

 the majority of the species, having for its type Clypeus Plotii, Klein. In the second, 

 the anal valley does not extend to the disc, but is separated therefrom by a greater or 

 less undepressed portion of test. The types of the second section are Clypeus Huyii, Ag., 

 and Clypeus subulatus, Young. 



All the species are limited to the Oolitic rocks, and they had their greatest develop- 

 ment in the seas which deposited the lower division of the Jurassic series ; as the Inferior 

 Oolite, zone of Ammonites Parkinsoui; the Bath Oolite group, and the Cornbrasli 

 contain the most of the species. I know of none in the Oxford Clay, and the Coral Rag 

 contains only one. 



