204 R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



trally, and the published figure was incorrectly oriented, as ascertained by 

 the introduction of the fourth column in the middle interambulacrum. 

 The type is in the Worthen collection at the University of Illinois, at 

 Urbana, Illinois (see foot-note, page 136). As figured it showed but 4 

 columns of interambulacral plates, being onl}^ a portion of a test, but, as 

 the authors say, it would very probably have had more columns if more 

 completely preserved. In the Museum of Comparative Zoology there is 

 a specimen of Rhoechinus burlingtonensis (catalogue number 3009) from 

 the same horizon and locality as the type. Besides other portions of the 

 test, this specimen has one interambulacrum nearly entire, and this 

 area in the upper portion has 6 columns of plates, showing that Meek 

 and Worthen were correct in supposing that the species might have more 

 than 4 columns of plates. 



The i)lates of the ambulacrum of the Cambridge specimen consist of 

 two columns, each of which is continuous across its own half area, as 

 shown by INIeek and AVorthen's figure. The species therefore distinctly 

 belongs to the genus Rhoechinus, as emended by Duncan (8). 



OBSERVATIONS ON PALJEECHINUS GIG AS AND RHOECHINUS ELEGANS. 



No species of the true genus Palmechinus as emended by Dr Duncan 

 has so far been discovered in America. 



Mr T. A. Jaggar recently saw in London, at the Geological Museum in 

 Jermyn street, a specimen of Palxcchinvs gigaf^, INI'Coy. By the kindness 

 of Dr E. T. Newton he liad an opportunity to study the specimen, which is 

 from the Carboniferous of Clitheroc, Lancashire. On plate 7, figures 38 

 and 39, are reproductions of Mr Jaggar's drawings, for which I am in- 

 debted to him. 



The interambulacrum of this species is described as having 6 columns 

 of plates, and such are shown in M'Coy's (28) original figure, which does 

 not, however, show the method of introduction of new columns. At the 

 ventral border of this specimen (plate 7, figure 38) we see tlie initial be- 

 ginning of the fifth column in pentagon 5. Plates below this area are 

 not preserved, but they would evidently below pentagon 5 consist of a 

 row of 4 plates, which would be members of columns 1, 3, 4 and 2 (the 

 last column, 2, in tlie figure is a restoration, as indicated by dotted lines). 

 In the second row above pentagon 5, the sixth column is introduced by 

 pentagon 6, with a heptagonal plate, H, on its left ventral border. The 

 position of columns 5 and 6 and the position of the heptagonal plate are 

 normal, according to the ideal method of arrangement as worked out for 

 Melonites. After the sixth no more columns are introduced in this species. 

 It is to be observed tliat these 6 columns were introduced relatively very 

 early in the life of the individual, so that when quite 3^oung it had at- 



