292 ROBERT TRACY JACKSON ON ECHINI. 



dorsally; pore-pairs uniserial. Interambulacra with in each area numerous (11 to 14) columns 

 of thin, small plates, imbricating aborally and from the center laterally. Primordial interam- 

 bulacral plates are in the basicoronal row; above this zone additional columns come in rapidly 

 in succeeding rows, showing a very accelerated development. The peristome is small, with 

 ten columns of ambulacral plates only. Oculars small, insert, covering the ambulacra, and 

 in part the interambulacra on either side. Genitals are large and high. The apical disc is 

 small. Periproct, as far as known, with many small, angular plates. The lantern is strongly 

 developed and inclined, as in other Palaeozoic species. 



The type species is H. beecheri sp. nov. from the Waverly Group of Warren, Pennsylvania. 

 This genus contains one of the most distinctive groups of known fossil Echini, remarkable 

 alike for the specialized ambulacra and interambulacra. The latter attain the largest number 

 of columns of plates known in the class, with a very accelerated development, yet all the plates 

 are retained, since there is no ventral resorption of the corona, and ventrally one can read stages 

 in development marked by the coming in of columns ; dorsally stages in senescence, as marked 

 by the dropping out of columns, without a single break. The flattened form of rarispinus, 

 the strongly pentagonal form of pentagonus and the bilaterally symmetrical form of beecheri 

 mark these as specialized and highly unusual sea-urchins (p. 225). 



I take keen pleasure in naming this genus for the late Professor Alpheus Hyatt, one of the 

 finest minds and finest natures that has graced science in this country, to whom I owe all that 

 a pupil can owe to a master. 



Key to the Species of Hyattechinus. 



Test flattened, nearly circular, eleven to thirteen columns of plates in each interambulacral area 



H. rarispinus (Hall), p. 292. 



Test pentagonal, fourteen columns of plates in each interambulacral area H. pentagonus sp. nov., p. 295. 



Test clypeastriform, flattened on the base, bilaterally symmetrical through an ambulacrum and opposite 



interambulacrum, eleven columns of plates in each interambulacral area . . //. beecheri sp. nov. p. 297. 



*Hyattechinus rarispinus (Hall). 

 Plate 21, fig. 6; Plate 22, figs. 1-8; Plate 23, figs. 1-7. 



Lepidechinus rarispinus (pars) Hall, 1868, p. 295, Plate 9, fig. 10; 1870 (revised edition), p. 340, Plate 9, 

 fig. 10; (pars) Keyes, 1895, p. 192; (pars) Jackson, 1896, p. 226, Plate 7, fig. 42; (pars) Klem, 1904, 

 p. 21; Lambert and ThieVy, 1910, p. 122. (See p. 290 and footnote, p. 394.) 



Lepidocentrus rarispinus Meek, 1874, p. 375. 



This interesting species is known from abundant material. The specimens are all internal 

 or external sandstone molds, very flat, without distortion or separation of plates, which indi- 

 cates that in life the test was much flattened, probably about as in the Recent Phormosoma, 

 or Echinarachnius. Plates are extremely thin; they are for the most part wanting, but some 



