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G'ewwA^— ACROSALENIA. Agassiz, 1840. 



This genus is composed of small or moderate-sized urchins with a thin spheroidal, 

 hemispherical, or depressed test. 



The ambulacral areas are narrow, straight, or slightly undulated, with two rows of 

 small crenulated and perforated tubercles on their margins, which diminish gradually in size 

 from the base to the apex. 



The inter-ambulacral areas are wide, having two rows of primary perforated tubercles, 

 raised on large prominent bosses with crenulated summits. 



The apical disc is proportionally smaller than in other genera of the Saleniad^, and 

 forms no prominence on the surface of the test ; the sur-anal plate, sometimes single, is in 

 general composed of many separate pieces placed before the anal opening, which renders 

 it excentral and displaces the vent backwards ; the anterior pair of genital plates are 

 larger than the posterior pair, and the single plate is small and crescentic ; the spongy 

 madreporiform body occupies the right anterior genital plate. 



The mouth opening is large, often one half the diameter of the test ; the peristome is 

 decagonal, with ten wide notches which have the margin reflected over their border. 



The primary spines are long, circular, flattened, or angular ; and they are sometimes 

 twice the length of the diameter of the body ; although apparently smooth, still with a 

 lens, their surface is seen to be covered with fine longitudinal lines. The secondary 

 spines are short, regular, round, and striated. 



The Acrosalenias are found in the different stages of the Oolitic rocks from the Lias to 

 the Portland, but they are most numerous in the lower division of the Oolites ; recent 

 researches have shown that this genus contains a much larger number of species than was 

 originally supposed, when many of them were erroneously referred to other genera, in 

 consequence of the apical disc being absent in a very great number of specimens. 



When the apical disc is absent, an Acrosalenia may be easily mistaken for a Hemi- 

 cidaris, but the aperture in Acrosalenia is always larger, and one of its angles projects 

 further into the single inter-ambulacrum than into the others ; the posterior pair of am- 

 bulacra are inore cm-ved backwards than the anterior pair, and the single anterior area is 

 always straight ; it requii-es a considerable practice of the eye and the handling of many 

 specimens, before we can determine accurately by these characters ; but practice has convinced 

 me of their value, and I have now no difficulty in distinguishing an Acrosalenia^ whether 

 the disc be present or not. The absence of true semi-tubercles from the base of the 

 ambulacral areas, likewise assists in the diagnosis. 



