CLASS I 



ECHINOIDEA 



305 



columns of isolated plates in each half-area at the mid-zone ; pore-pairs 

 multiserial. Three to eleven columns of plates in each interambulacral area. 

 This genus has fourteen species with a wide range of characters. The lowest 

 species, M. dispar (Fischer von Waldheim) has six columns of ambulacra! 

 plates, and is thus only one remove from Oligoponis. The highest species, 

 31. giganteus (Jackson) has twelve columns of ambulacral plates. At the 

 ventral border of the ambulacra there are typically four columns of plates 

 like the adult of Lovenechinus. From this stage passing dorsally isolated 

 plates first appear like Oligoporus, then additional columns, until the number 



Fio. 433. 



Melonechinus inultlponix (Norwood and Owen). Lower Carboniferous ; St. Louis, Missouri, 

 i/o natural size. B, Apical system, slightly enlarged (after Meek and Woithen). 



A, Test, 



characteristic of the species is attained. Dorsally some primary plates occur 

 next the oculars. Each species of Melonechinus presents developmental stages 

 in the ambulacrum like the adults of all lower genera, and lower species in 

 the family. The interambulacrum may have as few as three columns of 

 plates, as in M. obovahis Jack., which is the least known in the family, though 

 this species has ten columns of ambulacral plates ; or there may be eleven 

 columns of plates as in the extreme form, M. giganteus (Jackson). Between 

 the extremes every step is represented in the genus by developmental or 

 adult characters, or both. Plates of the test are often very thick, and usually 

 more or less strongly elevated melon-like ribs occur in both ambulacra and 

 iiiterambulacra, though these may be obsolescent or wanting, M. etheridgii 

 (Keeping). The peristome is known only in this genus for the family 

 (Fig. 371, D). Lower Carboniferous; Europe and North America. 



Lepidesthidae Jackson. 



Test elliptical, obovate, spherical or suhspheroidal. Two to twenty columns of 

 plates in each ambulacral area. Three to thirteen columns of plates in each inter- 

 ambulacral area. Plates imbricate. Primordial interambidacral plates in basicoronal 

 row. Base of corona not resorbed. Oculars usually all insert, genitals with one to 

 many pores each. Periproct j)lated with many thick plates. Peristome ivith many 

 rows of ambiUacral plates only. Primary spines with perforate tubercles, usually 



VOL. I X 



