380 ROBERT TRACY JACKSON ON ECHINI. 



The specimen which Professor Hambach described as Melonites irregularis I agree with 

 Dr. Keyes in considering referable to muUiporus. Hambach described irregularis as having 

 from six to ten columns of plates in an ambulacral area and from five to seven columns of 

 plates in an interambulacral area. This description is ambiguous, as I earlier stated (1896, 

 p. 240). Having studied the holotype (Plate 72, fig. 12), I find that there are ten columns 

 of plates in an ambulacral area at the mid-zone, as in muUiporus. In three interambulacral 

 areas there are seven columns of plates at the mid-zone, as is usual at this zone in muUiporus, 

 p. 377, but two areas are preserved ventrally only, and in these there are but five columns, 

 the ventral character of most species of the genus. The specimen is poorly preserved, and no 

 eighth columns were seen dorsally. Very likely none existed, as it is a small and presumably 

 young individual. There is no reasonable basis for separating it as a distinct species, but 

 rather it may be considered a small and immature muUiporus. Hambach's published figure 

 was incorrectly oriented as gathered from the ventral introduction of columns. 



The location of the type of M. muUiporus is unknown ; I am informed that it was proba- 

 bly destroyed by fire, a cast is in the Museum of Comparative Zoology, 3,226. This species 

 has been found in great abundance in the St. Louis Group, Lower Carboniferous, St. Louis, 

 Missouri. Slabs bearing twenty or more specimens occur in several museums, also a number of 

 choice specimens free from the matrix. Many specimens from this locality have been studied 

 in the Museum of Comparative Zoology, where there are some 75 specimens ; Yale University ; 

 Mr. Frank Springer's collection; Princeton University; Munich; Berlin; Strassburg; the 

 British Museum, and others. Keyes in localities for muUiporus, gives Clarksville and Char- 

 lotte, Tennessee. Miss Klem gives other localities taken from species she considers synonyms, 

 but which I treat as distinct. Troost also credits the species to Tennessee. All material I 

 have seen is from St. Louis, which is the only assured locality for the species. 



In my earlier paper (1896, p. 191, etc.) I worked out in part the development of the am- 

 bulacrum in Melonechinus muUiporus and compared the same with the structure in related 

 genera; but with more material and fuller knowledge additional facts and relations have 

 come to light. The ventral development, which is shown by the plates built in the youth of 

 the individual, presents a fairly complete series of stages, as shown in Plate 56, fig. 3 ; Plate 57, 

 fig. 1, area J; text-fig. 245. Close to the peristome there are typically four columns of plates, 

 which consist of wide occluded and narrow demi-plates in each half-area. This is like the 

 condition characteristic of Lovenechinus at the mid-zone, and also like the third stage in the 

 development of Oligoporus, as shown diagrammatically in text-fig. 237, p. 231. Proceeding 

 dorsally, in muUiporus (Plate 56, fig. 3) scattered isolated plates appear in the middle of each 

 half-area. This as a stage is comparable to the adult character of Oligoporus. Again, passing 

 dorsally, the isolated plates become frequent enough so that they form a vertical column (Plate 

 56, fig. 3). We have therefore in this arrangement a stage with six columns of plates which is 



