HYATTECHINUS. 295 



In area B the ambulacral plates of the dorsal side are brought into almost immediate contact 

 with those of the ventral side, superposing a narrower upon a wider plate. It is a similar con- 

 tact which I think accounts for the so called roofing plates of Sollas (1899, p. 705) in Palaeo- 

 discus, as elsewhere discussed (p. 251). In the internal view (Plate 23, figs. 2, 7) a series of 

 slanting pits exists between the inner pore and middle of the ambulacrum and represents molds 

 of the spinose processes, which extend into the test as described. The interambulacral areas 

 all show the primordial plate in place as I earlier described it (1896, p. 226). This plate is 

 directly comparable to the similar plate seen ventrally in Bothriocidaris, in the young of all 

 other Echini, and in the adults of types where it has not been removed by resorption in the 

 advance of the peristome. In the second row there are two plates in each area, and in the third 

 row three plates in each interambulacral area (Plate 23, fig. 1). In the fourth row there are 

 four plates, but in area E the fifth column is also represented by a plate in the fourth row, a 

 case of extreme acceleration of development. Columns 6, 7, 8, 9 come in rapidly, a new one 

 being added in each succeeding row as built. The mold of the lantern (Plate 23, fig. 2) shows 

 casts of the five interradial alveolar cavities, each of which was surrounded by two half-pyra- 

 mids, and also casts of the radial spaces between the half-pyramids seen best in areas B and D, 

 which were originally occupied by the interpyramidal muscles. Evidently the lantern was 

 inclined with the usual structure of the Palaeozoic lantern, as in Plate 27, figs. 4-6 (p. 22). 



Another specimen from the Yale Museum (Plate 22, figs. 3-6) shows certain differences 

 from that just described. The figures 3 and 5 are external molds of the ventral side, with tub- 

 ercles, spines, and peripodia. The figures 4 and 6 are external molds of the dorsal side of the 

 same individual specimens, and also show the surface characters of tubercles, etc. A mold 

 of the interior of the plates is shown in a small portion only of area J. In two areas of figure 

 4 and in a third area of the counterpart, figure 3, there are eleven columns of plates, as in tin- 

 type specimen. The impression of ambulacral plates on the peristome is very clear in figure 3, 

 and the mechanical displacement caused by the thrust of the lantern during flattening is clear 

 in figure 4. 



This material was collected by my friend, the late Professor Beecher, near his old home 

 at Warren, Pennsylvania. It is a pleasure to feel that his rare skill as a collector and preparator 

 is still contributing to the science and the principles that he loved so well. 



*Hyattechinus pentagonus sp. nov. 



Plate 19, fig. 8; Plate 24, figs. 1-4; Plate 25, figs. 1-i. 



The material of this species is a slab with several exquisitely preserved specimens which 

 Professor R. S. Breed kindly loaned me, and two additional specimens that I received through 

 the same gentleman. 



