FROM THE LOWER GREENSAND. 153 



in different individuals with age and the changing conditions of their existence. The 

 apical disc of Peltastes stellulatus, Ag., is one of these ; M. Cotteau remarks' 

 that the ovarial plates are sometimes almost smooth, and present hardly any isolated 

 points, as in P. pentaffo?iiferus, Gras. In certain examples the number of these points 

 augment sensibly, and the plates remaining perfectly smooth, the lines of the sutures are 

 punctuated {P. pundatus, Ag.). Instead of isolated points, the sutures are often marked 

 with lines or incisions which extend along the surface of the plates, and give to the apical 

 disc a very remarkable parsley-leaved aspect (P. stellulatus, Ag.). Sometimes this charac- 

 ter is exaggerated ; the incisions become deep, and penetrate even into the interior 

 of the plates, which unite at certain points, and produce a remarkable variety of disc seen 

 in certain specimens collected from the calcaires a Hchinospatagus cordiforniis in the 

 environs of Auxerre. In other examples, the plates independent of the points, more or 

 less elongated which exist on the suture, are bordered by little flexuous bourrelets, which 

 become attenuated as they approach the centre, as in P. Court audinus. Cot. These varie- 

 ties, when studied separately, as they were discovered by different observers, were 

 regarded as so many distinct types ; but a careful comparison of the series has taught M. 

 Cotteau that they are only so many varieties of the same species. The admirable figures 

 given of these variations fully confirm my learned friend's conclusions on the unity of the 

 species. The structure of the apical disc forms, therefore, an important character in the 

 history of this species ; and as the observations that have been made upon the difiierences 

 noted on the ornamentation and sculpture of its elements are applicable to other forms of 

 Peltastes, the careful study of this structure requires thoughtful notice in making a 

 diagnosis of all supposed new forms of Salenidae. 



The ambulacral areas are quite straight (fig. 1 h), and between the two lateral rows 

 of mammillated granules a zigzag line of microscopic granulets extends from the base to 

 the summit (fig. 3 h). 



The inter-ambulacral areas are wide and furnished with two rows of tubercles having 

 crenulated bosses and imperforate mammillons ; they are prominent, unequal in size, 

 largest at the ambitus, and decrease towards the peristome ; very large mammillated 

 granules fill up the miliary zone and form very regular circlets around the primary 

 tubercles. Between these granules in well-preserved specimens a number of microscopic 

 granulets are seen closely arranged around their base. 



The mouth-opening lies in a slight depression, and the peristome is divided into ten 

 lobes by feeble incisions, with reflected borders (fig. 2). 



The vent is transversely oval (fig. 3 a), and the periprocte has a slightly elevated and 

 inflated border. 



A variety of Peltastes stellulatus, Ag., figured in detail (PI. XLT, fig. 1 a — g), was col- 

 lected by the Rev. T. Wiltshire, F.G.S., from a greenish yellow band of the Red Chalk 

 at Speeton, Yorkshire. The true position of this specimen was between the upper por- 

 1 • Paleontologie FrancaisCi terrain Cretace,' t. vii, p, 102. 



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