FROM THE CHLORITIC MARL. 233 



HoLECTYPUS BiSTRiATUS, Wright, sp. IIOV. PI. LXV, fig. 3 a, h, c. 



Diapiosis. — Test sub-circular, sub-conoidal, depressed on the upper surface, and 

 flattened at the base ; ambulacra lanceolate, with four irregular rows of small tubercles ; 

 poriferous zones straight, narrow, with a smooth nude band extending from the disc to the 

 border on the inter-ambulacral side of each zone. Inter-ambulacra wide, plates narrow, 

 with horizontal rows of small tubercles on each, apical disc small. 



Dimemions. — Height eight tenths of an inch ; breadth one inch and eight tenths of 

 an inch. 



Description. — This rare Urchin was collected many years ago from the Chlontic 

 Marl, near Chard, by Mr. Weist, and kindly communicated for this work. It was 

 long considered to be a depressed variety of Discoidea cylindrica. In developing 

 the specimen, however, I displaced a portion of the test, which disclosed the 

 inner surface and the mould, and it then became evident that the Urchin was not a 

 Discoidea, but a true llolectypus, as it had none of the internal ribs at the ambitus which 

 distinguish Discoidea; it is the first Holectypus\}i\'&X,\\^% been recorded from the Cretaceous 

 rocks of England. 



The test is sub-circular, thin at the ambitus and depressed on the upper surface 

 (fig. 3 U) ; the ambulacral areas are lanceolate, with four or six rows of small tubercles 

 disposed in a zig-zag manner on alternate plates of the area which are very narrow, five 

 of them in vertical height being equal to one inter-ambulacral plate. The poriferous 

 zones are narrow, the pores small, and unigeminal, one pair of pores corresponding to 

 one ambulacral plate (fig. 8 c) magnified twice. 



The inter-ambulacral areas at the ambitus are nearly three times the width of the 

 ambulacra ; the columns are built of narrow plates, each supporting a horizontal row of 

 small tubercles, seven to eight in a row near the ambitus and fewer up the sides, each 

 tubercle is surrounded by a narrow areola, the boss of which is crenulated and the 

 summit perforated. The only specimen I have seen is the one under examination; 

 unfortunately, the surface of the test is so much rubbed that its minute structure can 

 only be made out by selecting those parts of the plates which are best preserved for 

 careful study with the glass. The tubercles are very small and numerous, and the 

 horizontal I'ows they form fill up the greater portion of the surface of the plates, so that 

 the number of miliary granules is inconsiderable. On each side of the inter-ambulacra, 

 separating the tubercular surface from the poriferous zones, two smooth nude bands 

 extend from the ambitus to the disc. These naked calcareous ribbons are very Avell 

 seen on one of the areas, and this bistriated structure forms a specific character of some 

 value and from which the specific name is derived. 



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