PLATE 23. 



Hyattechinus rarispinus (Hall). Page 292. 



Figs. 1-7. Waverly Group, Lower Carboniferous, Warren, Pennsylvania, collected by the late Professor C. E. Beecher; 



all are in Yale University Museum. 



Fig. 1. Yale Mus. Coll., 332. X 1.9. Same specimen as photograph, Plate 22, fig. 1. External mold of the ventral 

 side and in areas B, C, and D in part, an internal mold of the dorsal side. Ambulacra broad, petaloid ventrally, pore- 

 pairs uniserial. Interambulacra very clear; the primordial interambulacral plates are in the basicoronal row, two 

 plates in the second row, three in the third, four plates in the fourth row, and soon, the columns coming in very 

 rapidly, showing a highly accelerated development. The peristome is covered with many rows of low ambulacral 

 plates only (pp. 81, 213). In areas B, C, and D there exists in part an internal mold of the dorsal side; these 

 plates therefore have no tubercles. The ambulacral plates here are lower than they are ventrally and the areas 

 are markedly narrower. 



Fig. 2. Yale Mus. Coll. 333. Same specimen as photograph. Plate 22, fig. 2. X 2.7. Internal mold of the ventral side. 

 Ambulacral plates show pore-pairs as elevated plugs, also pits near the median line, which are molds of spinose 

 elevations extending proximally from the ambulacral plates. (Compare Plate 3, figs. 12, 13; p. 61.) Primordial 

 interambulacral plates are in the basicoronal row. In the center the mold of the lantern represents interradially 

 casts of the five alveolar cavities (each of which was enclosed by two half-pyramids) ; radially, casts of the spaces 

 between the half-pyramids, which in life were occupied by the interpyramidal muscles (compare Plate 5, fig. 2). 

 The lantern itself is quite absent. 



Fig. 3. Yale Mus. Coll., 325. X 1.6. Same specimen as photograph, Plate 22, fig. 8. External mold of the dorsal side. 

 Ambulacra are narrow, plates low. Interambulacral areas with 11 columns of plates in areas A and G, 12 columns of 

 plates in area I, and in two areas, C and E, 13 columns of plates (p. 50) ; part of the columns, notably 1 and 2, drop 

 out dorsally as a senescent character. Primary and secondary tubercles exist as sunken impressions. 



Fig. 4. Same specimen. Yale Mus. Coll., 325. X 3.7. Ambulacral detail enlarged; from the point marked X in fig. 3. 

 Pore-pairs are in peripodia. 



Fig. 5. Yale Mus. Coll., 327. External mold of a fragment of the dorsal side. X 5. Primary and secondary tubercles 

 on interambulacral plates. 



Fig. 6. Yale Mus. Coll., 329. Same specimen as Plate 22, fig. 5. X 5. External mold of the ventral side with spines 

 in place. 



Fig. 7. Yale Mus. Coll., 333. X 2. Same specimen as photograph, Plate 22, fig. 2. X 1.9. Internal mold of the 

 ventral side, showing pore-pairs existent as plugs, also pits which represent elevated spine-like processes on the proxi- 

 mal face of the ambulacral plates. (Compare Plate 3, figs. 12, 13; Plate 24, fig. 6; p. 61.) 



As the figures on this plate are all from drawings of external or internal sandstone molds, the orientation and lettering 

 are reversed from what they would be if taken from external or internal views of a test (pp. 20, 22). 



