222 ECHINODERMATA ECHINOIDEA SUB-BRANCH m 



small, numerous ; spines minute, acicular. Periproct circular ; basal plates 



FIG. 354. 



Melonites multipora, Norwood and Owen. Sub-Carboniferous ; St. Louis, Missouri. A, Test, */> natural size. 

 7J, Dorso-central system, slightly enlarged (after Meek and Worthen). 



with three to five genital perforations ; radials sometimes with a single pore. 

 Sub-Carboniferous ; North America and Europe. 



[As shown by Jackson and Jaggar (Studies of Melonitidae), the lAmb of Melonites 

 enter the peristomial margin as two plates. Passing dorsally, new columns are introduced, 

 rapidly at first, until the full complement is reached at or a little above the ambitus. The initial 

 plates of new columns are pentagonal, with a more prominent apex of the pentagon pointing 

 ventrally ; and an adjacent plate is characteristically heptagonal, thus facilitating the orienta- 

 tion of obscure fragments. Newly added plates near the dorsal area are more or less rhombic 

 in outline. The Amb terminate actinally in four plates ; and new vertical columns are added 

 between the median and lateral columns on either side, increasing dorsally.] 



Lepidesthes, Meek and Worth. Test large. Amb broad, with eight to 

 eighteen columns of small plates imbricating adorally ; and pores in single, 

 or occasionally in double pairs. lAmb with three or more columns of plates 

 imbricating aborally and laterally. Sub-Carboniferous ; North America. 



Hyboechinus, Worthen and Miller ; Pholidocidaris, Meek and Worthen. 

 Sub-Carboniferous ; North America. 



Family 3. Tiarechinidae. Zittel. 



Ambulacra narrow, with two vertical rows of plates pierced by a pair of pores. 



Interambulacra with a single peristomial plate, 

 followed by three vertically elongated plates only, 

 one on either side of the narrower median plate. 

 Basals large, with two genital perforations; 

 radials smaller, imperforate, and notching the 

 union of the basals slightly. 



Tiarechinus, Neumayr (Fig. 355). The test 

 of this unique genus is very small, flat actinally, 

 weeps, Laube. upper Trias; and sub- hemispherical dorsally. Below the 

 ambitus and actinally the ornament consists 

 of a plain primary tubercle to each plate ; else- 

 where the test is coarsely granular, including the very large apical system. The 

 solitary species, T. princeps, Laube sp., occurs in the Trias of St. Cassian, Tyrol. 



FIG. 355. 



Tiarechinus pri 



St. Cassian, Tyrol." "Ventral and lateral 

 aspects, highly magnified (after Loven). 



