408 ROBERT TRACY JACKSON ON ECHINI. 



*Perischocidaris harteiana (Baily). 

 Plate 65, figs. 1, 2; Plate 67, figs. 1-3. 

 (without name) Harte, 1865, pp. 67-69 and 90, Plate 5, three figs. 



.Irchacoeidaris sp. Baily, 1865c, p. 90. 



Archaeocidaris harteiana Baily, 1874, p. 42, Plate 4, figs, a-e; 1875, p. Ixviii. 



Perigchocidaris hartei Neumayr, 1881, p. 175, Plate 1, fig. 7; Lambert and Thiery, 1910, p. 120. 



Prosechinns hartciniana Pomel, 1883, pp. 113, 114, Plate [no number], fig. 35. 



Homotocchus hartii Sollas, 1892, p. 153. 



Perischodomus hartei Klem, 1904, p. 19. 



This species is known only from external sandstone molds of the dorsal side of two tests 

 (Plate 65, fig. 1). One of these is nearly complete, the other is a small fragment only. These 

 specimens on a single slab are in Trinity College, Dublin, where I studied them through the 

 kindness of Professor John Joly. A cast is in the Sedgwick Museum, Cambridge, England, 

 13. The mold of the more complete specimen (Plate 65, fig. I) is a cup-shaped cavity, a 

 plaster cast of which is shown in Plate 65, fig. 2. These indicate a spheroidal test, although 

 the ventral half is unknown. The diameter of the test is about 75 mm. The ambulacra at 

 the mid-zone are about 12 mm., the interambulacra about 35 mm. in width. 



The ambulacra at the mid-zone have six columns of ambulacra! plates composed of wide 

 occluded, narrow demi-, and one column of isolated plates in each half-area (Plate 67, fig. 1). 

 The plates of the two occluded columns are elevated in a high, rounded melon-like rib that 

 suggests the character in Melonechinus (compare Plate 54, fig. I), a feature not known in any 

 other species of the familj^, although the whole ambulacrum is rounded outward in a rib-like 

 fa.shion in Lepidesthes carinata (Plate 66, fig. 11). The pore-pairs lie in depressed valleys on 

 either side of the ambulacra and are situated near the outer border of each ambulacral plate. 

 A peripodium around each pore-pair is figured by Harte and better by Baily (1874), but I 

 overlooked it in my somewhat hurried examination of the specimen. Baily figures a small 

 tubercle on the inner part of each occluded plate. About fowr ambulacral plates equal the 

 height of an adambulacral, as Harte observed. 



The interambulacra are broad, with five columns of plates in each area in the zone of the 

 most adoral portion preserved, which I take it is about on the plane of the mid-zone (Plate 65, 

 fig. 1). Dorsally, part of these columns drop out before reaching the apical disc. As shown 

 in Plate 67, fig. 2, column 5 drops out first, then 4, and finally 3, columns 1 and 2 alone reaching 

 the apical disc, and the youngest plates lying in contact with the two oculars on either side. 

 This is the most extreme reduction of columns seen in any member of the Perischoechinoida 

 (p. 211). The interambulacra! plates are high and I'ounded outwardly, being more strongly 

 convex to the exterior than in any sea-urchin that I know. The plates are polygonal, but 



