28 ROBERT TRACY JACKSON ON ECHINI. 



A simple plate is one which has held its original character of one undivided and independent 

 piece. All plates in Palaeozoic Echini are of this character. , 



A compound plate is one made up by the coalescence of adjacent plates, seen in the ambula- 

 crum in certain groups, in which area alone is it known to exist. It is a character of the ambu- 

 lacral plates in the Centrechinoida and some Holectypina, but is unknown in the Palaeozoic. 

 The statement that compound ambulacral plates occur in Palaeozoic types has been made 

 (Duncan, 1889a, p. 13), but it is an error; no such plates occur in any known type. Occasion- 

 ally in regular Echini two genital plates, or a genital and an ocular, may fuse when the 

 resultant may in a sense be considered a compound plate (text-figs. 186, 195, 196, pp. 168, 169). 



Divided plates are those derived by the splitting on lines of solution of a previously 

 continuous plate. This has been shown by Mr. Agassiz (1904) in the genital plates of Phor- 

 mosoma. According to him (1904, p. 96), splitting occurs in interambulacral and ambulacral 

 plates in Phormosoma. Mr. Agassiz (1874, p. 642) attributed the formation of compound 

 ambulacral plates in the Centrechinoida, etc., to the splitting up of original plates. He also 

 says (1883, p. 17) that anal plates in Palaeozoic Echini are formed by splitting. I have had 

 this possible origin of plates in mind, but in my studies have seen no evidence of the origin 

 of plates by splitting in Palaeozoic or other Echini excepting genital plates in echinothuriids, 

 and rarely the same or ocular plates as variants (text-figs. 190-194, p. 169). 



When the antero-posterior axis is known, Loven's nomenclature of areas is adopted, num- 

 bering the ambulacra I-V and the interambulacra 1-5. In Palaeozoic species the axes are 

 rarely known on account of the usual absence of madreporic pores. The axes are known from 

 the presence of madreporic pores in Lovenechinus lacazei (text-fig. 240), Lepidesthes formosa 

 (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), and Meekechinus elegans (Plate 76, fig. 1). Axes 

 are assumed from bilateral symmetry in Hyattechinus beecheri (Plate 24, figs. 5, 8), from the 

 orientation of primordial ambulacral plates, Bothriocidaris archaica (Plate 1, fig. 1), and from 

 the apparently eccentric anal area, Echinocystites (Plate 20, fig. 1). In other Palaeozoic types 

 the axes are oriented arbitrarily by letters A to J, as discussed in the Introduction. 



Dr. Bather called my attention to a paper by M'Clelland (1840) in which he recognized that the name Diadema is not 

 available for a sea-urchin, but he did not propose a new name. To quote his words, p. 170, "We luckily get rid of the genus 

 Diadema [for Echini], from the term having been previously applied to a genus of Girrhopoda by Ranzani." 



High authorities reject Muerchen's (1774) names, therefore his Anademais unavailable. The name Calmarius annellaia 

 is given by A. Agassiz (1872, p. 104) as a manuscript name of Gray's, and is treated by Agassiz as a synonym of Diadema 

 selosum. As there is a closely related species, Echinolhrix calamaris {Diadema calamaris Gray), it seems probable that Gray's 

 generic name was Calamarius, taken from that species. Calmarius is evidently the same name as Calaniarius with the letter 

 a omitted by error; on this basis it is preoccupied by Calamaria Boie, 1827 (reptiles). 



As a new name is thus necessary for this genus, I propose Centrechinus (Ke'vrpov, a sting, and e'xivos, sea-urchin) 

 with C. selosus (Leske), from Key West, Florida, as the type. The Florida and \\est hulian Diademas are considered as 

 Diadema anlillarum Philippi by \. Agassiz and Clark (190S), but here antillarum is considered a synonym of selosum. 



As the name Diadema cannot stand for an cchinoderm, being preoccupied for a crustacean, the family and ordinal 

 names derived from that genus cannot by the rules of nomenclature be retained. They can be replaced by names from the 

 typical genus and may be called Centrechinidae and Centrechinoida. 



