318 HOLASTER 



The apical disc is small, long, and narrow ; and its elements are so closely welded 

 together that I have failed to detect the sutures which divide them (fig. 2 b). 



The under surface is moderately flat about the cheeks and side border ; it is ele- 

 vated in the middle by the prominence of the plastron, and the path of the postero-lateral 

 areas along the base is shown by a wide, smooth surface on each side of the central 

 elevation (fig. 2 a). 



The mouth-opening is small, situated a short distance from the border in a depression 

 formed by the incurving of the anteal sulcus. The peristome is transversely oval, and 

 placed about the junction of the first with the second fourth of the total basal length of 

 the test (fig. 2 a). 



The posterior border is very narrow, the anal area small, and the vent opens in its 

 upper part ; the oval periprocte is situated high up in this space, near the point where 

 the convex part of the back bends over towards the truncated portion of the posterior 

 border (fig. 2 <?). 



The surface of the plates is covered with a microscopic, close-set, miliary granulation, 

 beautifully preserved and shown in some of the specimens derived from the Grey 

 Chalk. On the larger plates a few small tubercles are arranged without much order, 

 from six to eight of these may be counted upon each of the plates (fig. 2 b). 



Affinities and Differences. — This species resembles some of the large forms of Hoi. 

 lavis, from which it differs very little ; the ambulacral summit is slightly subcentral, which 

 makes the direction of the ambulacral areas nearly uniformly quinque-radiate on the 

 upper surface, as seen in fig. 2 b. The anteal sulcus is shallow, the posterior border 

 narrow, and the position of the periprocte in the anal area is much the same in Hoi. 

 planus as in Hoi. Icevis. The rotundity and smoothness of the test, and the width and 

 shallowness of the anteal sulcus in Holaster planus form marked differences between it 

 and Hoi. suborbicularis, with which it has sometimes been confounded, and the same 

 characters serve to establish the differences between Hoi. planus and Hoi. subt/lobosus. 



Locality and Stratigraplbical Position. — This species is said to characterise a well- 

 marked zone of the Lower Chalk at Lewes and Dover, and I have specimens collected 

 from the Medial Chalk at Swafiham, Norfolk, sent by my old friend the late Mr. C. 

 B. Rose. It occurs also in the chalk with flints (in lower portion of the Upper Chalk) 

 near Lewes. The specimen (PI. LXXIX, fig. 1) from the Collection of the Rev. Thos. 

 Wiltshire, E.G.S., shows that the species was gregarious. 



Many geologists express surprise that several species of Echinides are recorded by me 

 as having been found in the Grey Chalk, Lower Chalk, Chloritic Chalk, and Upper 

 Greensand. There can be no doubt about the fact of the presence of this species 

 therein ; other forms also have often occurred to me when noting the distribution of 

 species in these beds. It may be suggested whether some artificial divisions, in the natural 

 groupings of the strata, have not been made on petrological rather than on palaeon- 

 tological grounds. If this conjecture should turn out to be correct then the Lower 





