LOVENECHINUS. 325 



occluded columns, which is a marked feature of the next genus, Oligoporus, and especially most 

 species of Melonechinus. The pore-pairs are biserial in sunken valleys on either side of the 

 ambulacral areas, and the pores are situated in that portion of each plate nearest the next 

 adjacent interambulacrum. All this as seen from the exterior; on the interior (Plate 43, figs. 

 4, 5) demi- and occluded plates are of about equal width, and pore-pairs lie near the middle 

 line of each half-area, also demi-plates which lie opposite horizontal interambulacral sutures 

 are higher and fan-shaped. Ventrally, next the peristome (Plate 42, fig. 1), ambulacral plates 

 built in youth are all primaries, with pore-pairs uniserial (as in adult Palaeechinus). Above 

 this zone plates are primary and occluded, with pore-pairs biserial (as in adult Maccoya), 

 before attaining the character plates of the genus. Dorsally, a comparable condition occurs 

 next the ocular in the placogenous zone (Plate 42, fig. 3) ; here ambulacral plates are primaries 

 with pore-pairs uniserial. Passing ventrally, the older plates are alternately primary and 

 occluded as localized stages in development, before they acquire the full characters of the 

 genus. Ambulacral plates bevel over the adradials laterally, and bear small secondary tubercles 

 and spines only. 



The interambulacra are wide, with from four to seven columns of plates in an area at 

 the mid-zone in the known species, composed of pentagonal adradial, and hexagonal median 

 plates. There are ventrally two plates in the basicoronal row, above which additional columns 

 come in as usual in the family (Plate 45, fig. 1). There is no imbrication in ambulacral or inter- 

 ambulacral plates, but the adradials bevel under the ambulacrals laterally. Interambulacral 

 and ambulacral plates bear secondary tubercles and spines only. Peristome and jaws are 

 unknown ; but they were doubtless similar to those of Oligoporus and Melonechinus. 



Oculars are usually all insert but one or more may be exsert. They are imperforate as 

 seen from the exterior, but indications of a pore on the interior have been seen in some cases 

 (Plate 41, fig. 2; Plate 42, fig. 6). Genitals are high, wide, with from three to five genital 

 pores each; a madreporite has apparently been observed in one specimen (p. 334; text-fig. 

 243, p. 331). Small angular periproctal plates, as usual in the family, have been seen in one 

 specimen (Plate 42, fig. 6). 



The type species is Lovenechinus missouriensis (Jackson). This genus differs from Maccoya 

 in that ambulacral plates are demi- and occluded instead of primary and nearly or quite occluded. 

 It differs from Oligoporus in that there are no scattered isolated plates in the middle line of 

 each half-area as in that genus, also as a correlated character in that pore-pairs are biserial 

 instead of multiserial. Lovenechinus in its development passes through a stage like the adult 

 of Maccoya, and Oligoporus in its development passes through a stage like the adult of 

 Lovenechinus (text-fig. 237, p. 231). It is thus a type structurally and phylogenetically inter- 

 mediate between Maccoya and Oligoporus. Lovenechinus includes certain species previously 

 referred to either Palaeechinus or Oligoporus, with two new ones (p. 304). 



