172 R. T. JACKSON — STUDIES OF PAL^EECHINOIDEA. 



Page 



Melonitidae 239 



Lepidesthidse 241 



ArchseocidaridEe 241 



Lepidocentridae 241 



Order III, Cystocidaroida 242 



Order IV, Plesiocidaroida 243 



List of publications quoted and referred to in the text 244 



Explanation of plates 246 



Studies of the Melonitid^. 

 introduction. 



In the followino; studies of the several families of the Palseechinoidea it 

 is the intention to take up for consideration the several genera and species 

 in their natural systematic order of sequence as expressed in the proposed 

 new classification (see table facing page 242). This intention is carried 

 out, as far as present knowledge admits, in families succeeding the Meloni- 

 tidffi. In the present family of the Melonitidse, however, the rule is de- 

 parted from, because the genera and species of which fullest knowledge is 

 attained are the more specialized, and the more primitive genera are least 

 known as far as available material goes. In this family, therefore, the 

 genera and species are taken up in the order of convenience for handling 

 the several cases rather than in the proper systematic order. The natural 

 systematic sequence of the genera would be, Rhoechinus, Palseechinus, Oli- 

 goporus and Melonites, as shown in the table facing page 242. For sources 

 of material, obligations due, and similar statements, the reader is referred 

 to the introductory, page 135, of the preceding paper : " Studies oiMelonites 

 multiporusy References to this earlier paper are frequentl}'' made by page 

 number, without further quotation, in the present paper. 



DESCRIPTION OF MELONITES GIGANTEUS, SP. NOV. 

 Plate 4, figure 19 ; plate 5, figures 21-24. Table, page 180. 



This new species. Melonites giganteus, is described from a single indi- 

 vidual, which is a truly superb, finely preserved specimen, from the 

 Lower Subcarboniferous of Bowling Green, Kentucky. It is in the col- 

 lections of the Museum of Comparative Zoology, catalogue number 2989. 

 The specimen is entirely silicified. The melon-like form is preserved 

 with hardly any distortion on the oral side (plate 4, figure 19), and on the 

 other side, though somewhat crushed, shows much structural detail. The 

 oral and aboral terminations of the ambulacra and interambulacra are 

 almost perfect in some areas, but orally they are wanting in the first row 

 of plates (plate 4, figure 19) ; corona very large, exceeding by far any pre- 

 viousl}^ described species of the Palseechinoidea ; height at the point of 

 greatest dimension, 11.5 centimeters ; height from oral to anal area through 



