PROM THE LOWER CHALK. 317 



(Eure) ; Laubressel, Sainte-Parre pres Troyes (Aube) ; Eourrain, Seigneley, Yonne ; 

 Sancerre (Cher) ; Cassis (Bouehes-du-Rhone) ; Vit pres Castellane (Basses- Alpes) ; 

 Bidart pres Biarritz (Basses-Pyrenees) ; Palarea (Nice) ; Altmann (Appenzell) ; Qued- 

 linburg, Prussia ; North Germany, &c. In Prance it has been collected from the zone 

 of Acanthoceras Rhotomagense, Brong., and in strata with Scap kites aqualis in Conde, 

 Sarthe, which is precisely the same zone from which our English specimens have been 

 collected. 



Holaster, l^evis, var. planus, Mantell. PI. LXXII, fig 2, a—f; PL LXXIX, 



fig. 1 a, b. 



Spatangus planus, Mantell. Geology of Sussex, p. 192, pi. xvii, figs. 9 — 21, 1822. 

 Holaster planus, Agassiz. Prodrom. Mem. Soc. de Neuchatel, t. i, p. 183, 1835. 



— — cTOrbigny. Paleontol. Fran^aise, Ter. Cret., t. vi, pp. 116,821, 



1853. 



— — Desor. Synopsis des Eckinides, p. 342, 1858. 



Diagnosis. — Test cordate, widest across the middle third, gradually diminishing in 

 width to the posterior border, which is narrow and slightly truncated, anterior border 

 a little depressed by the anteal sulcus. Ambulacral summit subcentral, upper surface 

 convex, gently declining all round towards the ambitus ; periprocte in the upper part of 

 the anal area. Under surface flat, plastron prominent, mouth-opening near the border 

 in a small depression, peristome transversely oval and bilabiate. Surface of the test 

 smooth. 



Bin tensions. — Antero-posterior diameter two inches ; transverse diameter one inch 

 and eight tenths ; periprocte above the base line seven tenths of an inch. 



Description. — This Urchin has a cordate outline, and is widest anteriorly, its greatest 

 transverse diameter being immediately behind the antero-lateral ambulacra ; from this 

 point it tapers gently to the posterior border, which presents a narrow truncation. 

 The upper surface is convex, and the vertical summit with the apical disc is subcentral ; 

 to this point all the ambulacral areas converge (PL XXII, fig. 2 b). The antero-lateral 

 pair are short, slightly bent, apetaloid, and lanceolate, they pass obliquely up the sides of 

 the test. The poriferous zones are narrow and superficial, the pores small and placed in 

 pairs ; there are forty pairs of holes in each of the avenues above the ambitus, which 

 are entirely invisible below ; the postero-lateral pair are equally oblique and lanceolate, 

 and their avenues of pores the same. In the figured specimen there are thirty pairs in 

 each avenue. The single ambulacrum is lodged in a wide, shallow anteal sulcus which 

 only slightly depresses the anterior border and entirely disappears in the upper half of 

 its length (fig. 2 a, b, d). 



41 



