26 PKOF. p. M. Duncan's revision of the 



Subclass II. EUEGHINOIDEA. 



Order I. CIDAROIDA (p. 24). 

 Family GmK-RTjiM, Agassiz Sf Besor, 1846, Gatal. Bais., Ann. 



Sci.Nat. vol. vi. p. 325 (pars). Joh. Miiller, 1854, Ahh. d. 



h. AJcad. d. Wiss. Berlin, p. 123. Desor, 1858, Synopsis, 



p. 2 (pars). Wyv. Thomson, 1874, Phil. Trans, vol. clxiv. 



p. 720. A. Agassiz, 1874, Revision, pp. 251 & 384 (pars)*. 

 Test spheroidal ; the ambulacra narrow, with two vertical rows 

 o£ very numerous low plates, which are primaries, and rarely 

 become compound, united by their edges, each with a pair of 

 pores arranged in single and rarely in double series ; other plates 

 continued from the peristome to the true mouth and imbricating, 

 besides being perforated ; interporiferous areas with large and 

 small granules only. 



Interradia broad, with few plates in two vertical series, most 

 with a large primary, scrobiculate tubercle, carrying a large 

 spine, secondary tubercles and granules with smaller spines ; 

 plates united at their edges ; some broad and low plates con- 

 tinued from the peristomial margin to the true mouth, and 

 imbricating. 



Apical system large, with five basal plates and five radial 

 plates, each with a perforation ; periproct in the midst, covered 

 with plates. A madreporite ia the right anterior basal plate. 



Jaws with the foramen of the pyramid small, and not closed 

 in by epiphyses ; teeth grooved ; perignathic girdle discontinuous, 

 with a bifid process on each interradium only. Branchiae in- 

 ternal; tentacles subheteropodous f- Peristomial branchial inci- 

 sions and external branchiaB absent. Spines variable. Spheridia 

 absent. 



* A. Agassiz pointed out in the ' Revision ' that the anatomical researches of 

 Johan. Miiller rendered a correct definition of the family Cidaridse possible. 

 But nearly all the definitions which have been recorded have been too synthetic. 

 Since A. Agassiz gave such a mine of information regarding the recent genus 

 Cidaris and its subdivisions, there has been a general movement towards simpli- 

 fication. The old subfamily " Salenidm " has been eliminated and the fossil and 

 recent Cidaridss form a tolerably homogeneous group. In the Report on the 

 ' Challenger ' Echini, 1881, A. Agassiz remarks concerning the branchial slits at 

 the true mouth of recent specimens of Cidaris (p. 53) : — " Whether it is these 

 organs (gills) which find their way through the cuts or not, in our Florida 

 species, I am unable to state ; and a renewed examination of living specimens 

 will be necessary before we can settle this interesting question." 



t The definition of terms is given at the end of this Essay. 



