1872.] 



30LLAS — UPPER-GREENSASTD VENTRICULITES. 



65 



spherical bodies are found from yutt" to j^-" diameter, which agree 

 in their mineral characters with the opaque yellowish-brown fibre 

 of No. ii. 



The Hexaradiate Elements. — The Ventriculite skeleton is formed by 

 the regular anastomosis of a number of hexaradiate elements, each 

 one of which consists of six radial fibres, apparently tubular, 

 diverging from a common centre at right angles to each other. To 

 the end of the first quarter of their course the fibres are of the most 

 microscopic diameter ; but they then suddenly expand, their diameter 

 becomes many times greater, and continues so to the end of their 

 entire course, where they open into the similar radii of surrounding 

 elements. At their point of expansion they give off four fibres of a 

 diameter of about 2 5 1 00 ", which pass backwards at an angle of about 

 45° to join similar fibres given off by the other arms of the same 

 element, so as to form a skeleton octahedron about the common 

 centre (fig. 2). This is the typical structure of the hexaradiate 



Fig. 1. 



Fis. 2. 



B, radii. 



r, interior radii. 



o, spaces. 



elements. As in the Vitrea, however, abnormalities occur, the ele- 

 ments sometimes becoming pseudo-heptaradiate (fig. 3), and some- 

 times apparently triradiate. The additional ray of the heptara- 

 diates is prolonged merely from one of the octahedral fibres, and 

 does not start from the common centre of the six normal rays. 

 The triradiates also have nothing in common with the hexaradiate 

 elements. 



Combination of the Hexaradiate Elements. — Considering one of these 

 elements, it will be seen that each of its four horizontal radii is 

 freely continuous in the same straight line with a horizontal radius 

 furnished to it by one of four other elements symmetrically disposed 

 in the same plane around it ; each of the vertical radii is continuous 

 in the same way with a vertical radius derived from each of two 

 other elements, one placed above and the other below it : all the 



VOL. XXIX. — PARTI. F 



