of the Chalk. 93 



of that very beauty and regularity, but in the exquisite delicacy 

 of a further texture, of which the Euplectella does not possess a 

 trace, and which, so far as I can learn from the best authorities, 

 has never been hitherto observed in any object, animal or vege- 

 table, recent or fossil. 



Through the kindness of Professor Owen and the liberality of 

 the proprietor of the specimen, Mr. Broderip, I have had an op- 

 portunity of carefully examining the Euplectella. While I am 

 thus enabled to speak to the fidelity of Prof. Owen's description, 

 I can also speak with more assured certainty as to points to 

 which, having the structure of the Ventriculidse before my eye, 

 my attention was more particularly directed. 



The Euplectella is composed of an arrangement of fasciculi of 

 fibres, one course over the other, not anastomosing together : its 

 principal substance consists of two layers, a longitudinal and a 

 transverse one, of which the former is external to the latter. 

 These, thus crossing at regular intervals, form the regular tex- 

 ture which excites Prof. Owen's just admiration, — an arrange- 

 ment in which they are held by the interlacement of other and 

 more delicate fibres tied by the same fairy fingers which Dr. John- 

 ston describes as knitting the plexus of one of his delicate zoo- 

 phytes. The squares thus formed are about the eighth of an inch 

 in size, — gigantic in comparison with the squares filling the 

 membrane of the Ventriculidse. 



The membrane of the Ventriculidse is composed of very de- 

 licate fibres arranged in squares, of which the larger ones measure, 

 on an average, considerably less than the 100th of an inch on each 

 of their sides *. The fibres of the Ventriculidse are not arranged in 

 fasciculi, nor in layers the one overlying the other concentrically or 

 otherwise. Neither are they at their points of crossing wrapped 

 together by other interlacing fibres. The substance of the Ventri- 

 culites simplex (see PI. VIII. fig. 1), which is the true type of the 

 whole family, is not quite one line in thickness (about the sixteenth 

 of an inch). It consists of five thicknesses of the squares 1 have 

 mentioned. Consequently the substance of the body of the ani- 

 mal consists of a membrane composed of an exceedingly delicate 

 fibre anastomosing in every direction, so as to form both in the 

 plane of its surface and of its thickness regular squares (see 

 Plate VII. fig. 8). As the body increases in size from the base to 

 the upper margin the fibres increase in number, and this takes 

 place, not, as in the Euplectella, by the " convergence and inter- 

 blending of contiguous fibres," but by the addition of a fresh fibre, 

 generally at the middle of the outward boundary fibre of a 



* The sizes are pretty constant in different species. There is some va- 

 riation however. Those described are the largest. The relative dimensions 

 appear constant. 



