. FROM THE UPPER GREENSAND. 157 



there are four or five tubercles in each series, and of these the two uppermost only attain 

 a considerable development (fig. 1 b, and fig. 1 d, and fig. 3). The others are much 

 smaller, and diminish in size as they approach the peristome (fig. 1 c and fig. 1 d) ; the 

 margin of each plate is surrounded by a series of small mammillated granules (fig. 1 <?, 

 and fig. 3), which form a circle around the tubercles, and in the upper part of the area 

 between the larger granules a number of smaller granulets are placed between tliem 

 (fig. I d), thus completing the ornamentation of the miliaiy zone. 



The base of this urchin is flat, and the mouth-opening small (fig. I c) ; the peristome 

 is divided into ten nearly equal lobes, by feeble incisions (fig. 1/and fig. 1 c). 



The apical disc is very large, and occupies all the upper surface of the test (fig. 1 b, 

 fig. 3). Its ovarial plates are deeply cut, with numerous incisions, which are much better 

 defined by figures than words ; faithful portraits of these most complicated impressions 

 my friend Mr. Bone has given in fig. 1 b, fig. 2, fig. 3, and fig. 4. These incisions are 

 large and deep, and extend through the thickness of the plates ; three deep incisions mark the 

 line of suture between the ocular and ovarial plates (fig. 2) ; the two antero-lateral ovarials 

 have another series of impressions of a like character before the vent and behind 

 the oviductal opening. The oviductal holes are large ; the two anterior are placed 

 forward on their respective plates ; the three posterior lie near the inner borders of 

 the genital plates and opposite the angles of the periprocte. The ocular plates are heart- 

 shaped, and the orbit occupies a depression at the summit of the ambulacra, near the 

 extreme border of the plates. The vent is large and transversely oblong, sometimes 

 even becoming angular (fig. 1 b). The periprocte is elevated, and its funnel-shaped 

 extremity encircled by a bourrelet directed backwards (fig. 3). Most specimens show 

 this elevation of the vent, but in many varieties it is not so conspicuous as in the type 

 specimen figured in fig. 3, where the test is magnified twice. The spines are long and 

 needle-shaped (PL XLII, fig. 4) ; above the milled ring is a broad band of longitudinal 

 hues, and the rest of the stem is covered with very fine longitudinal microscopic lines. For 

 further details on the spines of this and other species, see description of PL XLII. 



Affinities and Differences. — Peltastes clathratus is readily distinguished from its con- 

 geners by its subglobose test, the great size of its apical disc, and the number, width, and 

 depth of the numerous incisions that divide its surface ; its flat base and small mouth 

 opening, with its rounded sides, present an ensemble of characters which well characterise 

 the species and separate it from all the others. 



Locality and Stratigraphical Position. — This species was in former years veiy common 

 in the Upper Greensand near Warminster. Small examples were most abundant, and 

 large specimens were rare. The largest tests are collected from the Grey Chalk near 

 Folkstone ; my kind friend the Rev. T. Wiltshire has obtained several large examples from 

 this stratum, and several of these have their spines in situ on the test. 



M. Cotteau gives as the French localities of this species, -where it is very rare, I'fitage 

 Cenomanien, Le Havre, Seine-Inferieurc, La Perriere, Orne, Craie a Scaphites. 



