HYATTECHINUS. 293 



fragments of plates in Yale Museum specimens are only a small fraction of a millimeter in 

 thickness. The test was nearly circular in outhne but slightly elongated through an ambula- 

 crum and opposite interambulacrum, as in Plate 22, figs. 1, 8, indicating a tendency to the 

 bilateral symmetry which is more marked in H. beecheri (Plate 24, figs. 5-8). The ambulacrum 

 is broad, petaloid ventrally, at its widest part twice the width of the same area at the mid- 

 zone. Ambulacral plates are relatively high, especially ventrally, where one and one-half 

 about equal the height of an adradial plate; dorsally about two and one-half equal the height 

 of an adradial. This shows that the younger ventral plates are higher, which I believe is a 

 more primitive character than that seen in the lower, later built dorsal plates. Pore-pairs 

 are uniserial, in well marked peripodia, and each pair ventrally is on an angle, with the outer 

 pore distinctly higher than the inner. Ambulacral plates bear small secondary tubercles only, 

 and ventrally on the interior (Plate 23, figs. 2, 7) each plate bears a spinose projection that 

 extends into the interior of the test. This spinose projection lies between the inner pore and 

 the middle of the area; it is otherwise known amongst fossils in H. beecheri only, as here described, 

 and amongst Recent Echini can be directly compared with the internal spines that I show 

 in Phyllacanthus and Eucidaris (Plate 3, figs. 12, 13; p. 61). 



The interambulacra have from 11 to 13 columns of plates at the mid-zone, this range being 

 shown in a single specimen in its several areas (Plate 23, fig. 3). The type specimen, however, 

 (Plate 21, fig. 6), has 11 columns in each of the three areas preserved. The primordial inter- 

 ambulacral plates are in place in the basicoronal row (Plate 23, fig. 1), and new columns come 

 in rapidly, one, or sometimes two columns appearing in each row as added up to the ninth, but 

 later columns may come in more slowly. Dorsally, the adambulacral or even additional columns 

 drop out on approaching the apical disc, indicating senescence. The interambulacral plates 

 bear a small eccentric primary tubercle with secondary tubercles (Plate 23, figs. 5, 6), to which 

 are attached corresponding small spines. The peristome is covered with ten columns of ambula- 

 cral plates only (Plate 23, fig. 1), much as they are in a Recent echinothuriid (text-fig. 43, p. 80). 

 I (1896) first showed these rather imperfectly in rarispinus, and Mortensen thought it made a 

 connection between the two genera, but I cannot agree with this view, as elsewhere discussed 

 (p. 213). The ocular, genital, and periproctal plates are unknown in this species, but they were 

 probably similar to those of pentagonus. A powerful lantern is represented by molds of the 

 parts (Plate 23, fig. 2) in several specimens. The pyramids are wide-angled, and therefore 

 the lantern is inclined, as in other Palaeozoic types. 



Waverly Group, Lower Carboniferous, Meadville, Pennsylvania; the holotype (Plate 21, 

 fig. 6; Plate 22, fig. 7) is from this locality, and is in the American Museum of Natural History, 

 no. 6,392. Same horizon, Warren, Pennsylvania; a superb series of ten specimens largely col- 

 lected by the late Professor C. E. Beecher, is in the Yale University Museum nos. 324-333; 

 another fine series of six specimens from the same locality is in the New York State Museum 

 no. 1,799. 



