302 ROBERT TRACY JACKSON ON ECHINI. 



as usual, but shows no bifid distal end; probably it was worn off, but possibly no bifid tip existed, 

 as in some Recent Echini. In the ventral view (Plate 27, fig. 4) the lantern is in part covered 

 with the ambulacral plates of the peristome, which alone fill this area in the species and the 

 family. It is interesting to see how near the lantern of this ancient t3^e is to the character 

 of the lantern in young modern regular Echini, as discussed (p. 181), the similarities being in 

 the inclined angle, shallow foramen, and wide spaces for long interpyramidal muscles. 



FamUy PALAEECHINIDAE M'Coy. 

 Palaechinidae M'Coy, 1849, p. 253; Lambert and Thlery, 1910, p. 119. 

 Palaeechinidae (pars) Loven, 1874, p. 40. 



Mdonitidae (pars) Zittel, 1879, p. 484; (pars) Duncan, 1889a, p. 15; Jackson, 1896, p. 239. 

 Archaeocidaridae (pars) Duncan, 1889a, p. 8. 



Mclonechinidac Lambert, 1899a, p. 53; Lambert and Thiery, 1910, p. 119. 

 Meleehidae Lambert and Thiery, 1910, p. 120. 



Test high, spheroidal, elliptical, or obovate; ambulacra narrow or wide, with from two 

 to twelve columns of plates in each area at the mid-zone. Two pores to each plate, situated 

 in a peripodium. Pore-pairs uniserial, biserial, or multiserial. Interambulacra wide or rarely 

 narrow, with from three to eleven columns of plates in each area at or dorsal to the mid-zone. 

 Plates moderately to very thick, not imbricating, but ambulacral plates bevel over the adradials 

 on the marginal sutures. The base of the corona is resorbed so that there are two plates in the 

 basicoronal row in each interambulacral area, instead of one plate as in the Lepidocentridae. 

 Tubercles and spines are secondaries only, on all plates of the corona, a feature pointed out by 

 M'Coy. Peristome (known only in Melonechinus, Plate 5G, figs. 7, 8) with many rows of 

 ambulacral and some interradially situated non-ambulacral plates. The apical disc is small, 

 the diameter in the specimens measured being about 13 to 25 % of the diameter of the test, 

 usually it is proportionately smaller than in most Recent regular Echini (pp. 87, 104). Ocu- 

 lars are insert, or rarely one or more to all may be exsert without pores as seen from the exterior. 

 Genitals large, high, with from two to five pores each; a madreporite is rarely recognizable. 

 Periproct with many angular plates. Lantern inclined, rarely preserved, but as far as known, 

 with the typical character of the Palaeozoic. The characters of the ambulacra at the mid-zone 

 are the essential basis for the division of this family into genera and in part into species. The 

 characters of the genera in brief are given in the key (p. 207), and their phylogenetic relations 

 are set forth in the systematic table (p. 209) and in text-fig. 237, p. 231. (See pp. 82, 362, 414.) 



The family Palaeechinidae was formed by M'Coy (1849) to include Palaeechinus and 

 Melonechinus, the only genera then known. He defined the group clearly, but his earlier 

 name has largelj' given place to Zittel's name IVIelonitidae. M'Coy's genus Palaeechinus 

 included species that have since been generically divided with some resulting confusion, and 

 some new types are generically different from any species known to M 'Coy ; therefore it is 

 necessary at this place to give the history of the subject. 



