404 ROBERT TRACY JACKSON ON ECHINI. 



to seven pores each, but one of the plates does not show any pores (Plate 64, fig. 8). In the 

 periproct are several plates, not shown in the figure; they a-re not in contact, and are turned 

 on edge, but apparently belong to this area. They are small and angular, similar to the peri- 

 proctal plates of Lepidesthes (text-fig. 251, p. 428). Another specimen from Hook Head in 

 the Sedgwick Museum, Cambridge, England, no. 16, has interambulacral plates showing pri- 

 mary and secondary tubercles well, and also primary and secondary spines. The primary 

 tubercles are perforate, with clearly defined scrobicules, and are situated eccentrically. There 

 is one primary tubercle to each adradial plate and also on some plates of the median columns. 

 The secondary tubercles are irregularly scattered over the surface of the plates. Primary 

 spines are stout, swollen at the base, and measure about 11 mm. in length. The secondary 

 spines are small, filiform, and measure about 3.5 mm. in length. 



In Trinity College, Dublin, are two fine specimens of Perischodomus, which I studied 

 through the kindness of Professor John Joly. There is no locality label, but, judging from the 

 lithological character, they are probably from Hook Head, County Wexford. One of these 

 specimens is a dorsal external view (Plate 62, fig. 7; Plate 64, figs. 3-5). The ambulacra are 

 narrow. Dorsally, all plates cross the half -areas, and pore-pairs are uniserial (Plate 64, fig. 5) . 

 At the mid-zone (Plate 64, fig. 4), all the plates meet the middle of the area, but some of them 

 are narrowed outwardly and more or less completely shut out from interambulacral contact. 

 As an associated feature, the pore-pairs are somewhat biserial. There are five columns of 

 interambulacral plates at the mid-zone in each area (Plate 64, fig. 3), but dorsally in each area 

 columns 3 and 4 drop out before reaching the apical disc. Columns 1, 5, and 2, however, 

 extend to the upper limits of the areas. The plates are marginally rounded and strongly 

 imbricating. Each adambulacral plate bears an eccentric primary tubercle, and some of the 

 median plates also have a primary tubercle as seen in Plate 62, fig. 7, but they were overlooked 

 in making the drawing of this specimen. 



The second specimen which I saw in the Trinity College Collection at Dublin is an internal 

 view of the ventral side, and is very choice as the only specimen known in this species showing 

 the ventral portion of the test (Plate 62, fig. 6; Plate 64, fig. 2). The ambulacral plates 

 are very much wider than on the exterior. This is ascribed to the lateral beveling of 

 ambulacral plates under the adradials, but, as the exterior of the ventral side is unknown, it 

 may be in part due to a greater width of the ambulacrum ventrally. As seen in this internal 

 view, the ambulacral plates imbricate aborally and bevel over the adradial plates, the reverse 

 condition of that seen from the exterior (compare text-figs. 32, 34, p. 75). The pore-pairs are 

 about in the middle of each plate, a usual character of the interior (compare Plate 20, fig. 10). 

 Adorally some ambulacral plates extend beyond the primordial interambulacral plates on to 

 the peristomal area and by their position (Plate 62, fig. 6) indicate that ambulacral plates alone 

 existed on the peristome, as in other members of this family and all of the Lepidocentridae 



