MELONECHINUS. 



species in the genus, the apical disc is relatively smaller, its diameter being about 13 to 16 % of 

 the diameter of the test (pp. 87, 104, 339). The apical disc is composed of five oculars and five 

 genitals. The oculars separate the genitals, reaching the periproct, are imperforate, and ven- 

 trally cover the ambulacra and laterally the interambulacra in part on either side. No excep- 

 tions to these statements are known to me in the genus (Plate 56, fig. 6). The genitals are 

 high and wide, presenting one face to the periproct, one to each ocular on either side, and 

 ventrally a rounded face to the interambulacrum, without reaching the ambulacra on either 

 side (Plate 53, fig. 1; Plate 56, fig. 1). It is often difficult to see the interambulacral plates 

 extending quite to the oculars, but, barring something abnormal, I believe this is the character 

 (p. 89). Genital plates in this genus have typically three or four pores each. Occasionally 

 only two, or one, or even no pores are visible (p. 149). When one or none are visible, it is 

 doubtless a case of obliteration of pores in fossilization. I have seen no case of madreporic 

 pores in the genus, though such are figured by Keyes (1894) in M. multiporus. His observa- 

 tion is further considered under that species. The plates of the periproct strangely enough are 

 known in only one specimen, an external mold (Plate 56, fig. 1). They are small, angular, 

 and evidently fill the area as in Palaeechinus and Maccoya. The lantern is inclined, pyramids 

 wide-angled, with moderately deep foramen magnum, and the teeth extend a considerable 

 distance beyond the pyramids ventrally. 



Mr. Agassiz (1892, pp. 72, 73) expresses the view that "the many pores found on the genital 

 and ocular plates of the Palaechinidae " are "only perforated tubercles, or the pits left after the 

 minute radioles have been broken off." This family does not have perforate tubercles on any 

 part of the test, and the pores pass directly through the genital plates, as the perforations of 

 tubercles do not. I believe no one has shown supernumerary pores in ocular plates in this 

 family excepting Baily, and he was mistaken (p. 309). I have never seen any pores perforating 

 ocular plates in the Palaeechinidae. Mr. Agassiz (1881, p. 12) says that many genital plates 

 may be occupied by the madreporite in the Palaeechinidae as shown in several genera figured 

 by Baily, Worthen, and others. I know of no case in the Palaeechinidae in which a madre- 

 porite has been previously figured excepting by Keyes in Melonechinus, and I know of no case 

 in this or any other family of Palaeozoic Echini in which madreporic pores have been shown in 

 more than one genital plate (p. 172). 



Considering the lantern of Melonechinus multiporus as figured by Meek and Wort hen 

 (1866), Mr. Agassiz (1874, p. 647) says that there is a furrow in the center of the teeth, resem- 

 bling that of the teeth of Archaeocidaris as figured by Miiller (1857) (my Plate 10, figs. 6, 7). 

 Neither Meek and Worthen nor Muller as far as I am aware figured teeth, but they did figure 

 dental pyramids, and the "furrow" referred to by Mr. Agassiz is evidently the median suture 

 of the pyramids. Melonechinus does not have a furrow on the exterior of the teeth though 

 doubtless there was a groove on the inner side as in other members of the Perischoechinoida 

 (text-figs. 207, 208, p. 184). 



