500 MONOGRAPH OF THE ECHINODERMATA. 



secondly, the Echini, possessing only one aperture at the 

 base. In 1784, Klein published his 'Dispositio Naturalis 

 Echinodermatum,' which added considerably to the know- 

 ledge of these fossils. He classed them according to the 

 situation of the mouth and vent, a plan which has been con- 

 tinued, and in part adopted, in every succeeding arrangement. 

 In dividing them into classes, sections, and genera, he 

 made use of the following nomenclature : — 



EMMESOSTOMI.— Mouth in the centre of the base. 



APOMESOSTOMI.— Mouth out of the centre. 



And the further subdivided classes were, — 



Anocysti. Vent in the upper part. 

 Cidaris, Clypei. 



Catocysti. Vent in the under part. 

 Fibula, Cassides, Scuta, Placenta. 



Pleurocysti. Vent in the side. 



Subsequently Breyne, Van Phelsum, and Leske, assuming 

 for a groundwork the sections of Klein, proposed a different 

 ent classification ; — the first reducing the genera to seven, — 

 the second forming them into twenty, — and the last again 

 curtailing them to ten. Lamarck, however, instituted consi- 

 derable alterations; and without overlooking, as a primary 

 character, the position of the mouth and vent, he established 

 generic distinctions from the size and form of the ambulacra. 

 The following is an outline of his division. 



1. The vent below the margin ; in the lower surface ; or in the margin 

 * The mouth beneath, always central. 



Scutella. 



Clypeaster. ■ Ambulacra contracted. 



Fibularia. 



EchinoneusA Ambuhcm comp i e te. 



Cralerites. ) r 



** The mouth beneath, not central, but approaching the margin. 

 Ananchites. Spatanyus. 



2. The vent above the margin, and consequently dorsal. 



a. The vent dorsal, but approaching the margin. 



Cassidulus. Nucleolites. 



b. The anus dorsal and vertical ; the shell regular. 



Echinus. Cidarites. 



Several authors have since introduced various modifications 



