FROM THE WHITE CHALK. 301 



Cardiaster PYGMiEUS, Forbes. PI. LXIX, figs. 1 a — e. 



Cardiaster PYGM.a:us, Forbes. Ann. Nat. Hist , 2nd ser., vol. vi, p. 444, 1850. 



— — Forbes. Mem. Geol. Surv., decade iv, pi. ix, notes, 1852. 



— — Morris. Catal. of Brit. Foss., 2nd ed., p. 73, 1854. 



Diagnosis. — Test very small, ovate, and cordate ; anterior channel large ; ambulacral 

 areas wide, sparsely covered with microscopic tubercles ; posterior border truncated 

 vertically ; periprocte in the upper third ; upper surface gently rounded, with an elevated 

 carina between the apical disc and the upper margin of the periprocte. 



Dimensions. — A middle-sized specimen measured in the antero-posterior diameter 

 half an inch ; in the transverse diameter five twelfths of an inch, and in height four 

 twelfths of an inch. It is found, however, that the proportions vary considerably. 



Description. — This small Cardiaster is common in the Chalk at Dover, where many 

 good specimens were collected several years ago by Major C. E. Cockburn, R.A., who 

 kindly supplied examples for this work. The outline of the test is ovate, gently rounded 

 on the upper surface (PI. LXIX, figs. 1 d, e), and provided with a subcarinated ridge, 

 which rises behind the apical disc (fig. 1 a), and extends backwards to the upper border 

 of the periprocte (fig. 1 d). The anteal sulcus is wide (figs. 1 a, e) and strongly 

 carinated, and the posterior border is sharply truncated (fig. 1 c) ; the oval periprocte 

 opens in the upper third (fig. 1 d) ; the sides of the test are moderately inflated (figs. 1 e 

 and 1 c), and the view from the base shows the ovato-cordate ambitus of this beautiful 

 little Urchin (fig. 1 6). 



The ambulacral areas are acutely lanceolate on the upper surface, the antero-lateral 

 curve a little backwards, and the postero-lateral pass direct to the disc ; the poriferous 

 avenues are very narrow, and the pores in single pairs are set closely together. 



The surface of the test appears to be perfectly smooth. When examined with a lens, 

 however, it is seen to be covered with minute granules, among which a number of 

 primary tubercles of small size are scattered (fig. 1 a, b, c). 



The base is flat at the sides, and has a central elevated ridge, which extends from 

 the peristome to the posterior border (fig. 1 c, b) ; the mouth is transversely oblong, 

 and situated near the anterior border (fig. 1 b). 



Affinities and Differences. — This Urchin when magnified two diameters, as it is in our 

 figures, has a striking resemblance to Hoi aster lavis ; but differs in having the anteal 

 Sulcus deeper and wider, its sides more angular, and the posterior border broader and 

 more sharply truncated than in that species ; the mouth is also nearer the anterior border. 



Locality and Stratiprqjhical Position. — This small Urchin has hitherto been found 

 only in the Upper White Chalk at Dover, Kent, and was first briefly noted by the late 

 Prof. Edw. Eorbes, E.R.S., in the ' Ann. and Mag. of Nat. History,' 2nd series, December, 

 1850. 



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