172 OEIGINAL AETICLES. 



size, in tlie substance of a number of tlie spines. In water, or in 

 spirit, tlie tubes stand out with a rounded opaque appearance from 

 tlie clear matrix. They become clearer on the addition of liquor 

 l^otassce ; but when a diy section is placed in turpentine or Canada 

 balsam, they are rendered quite transparent \ so much so, that it is 

 advisable, before adding turpentine, to steep the specimen in a 

 potash solution of carmine, which dies the tubes, and leaves the 

 matrix uncoloured, save only at the part bounding the deep aspect, 

 where, there being less mineral deposition, and consequently a softer 

 texture, the staining is nearly uniform. 



The clearing up of the tubes, on addition of turpentine, brings 

 into view in their interior, a great number of long, and generally 

 somewhat pointed crystals, lying for the most part transversely in 

 them, or nearly so. They are usually broader in the middle than at 

 their extremities, are not always perfectly straight, and are always 

 distinct from one another. Probably they are composed of carbonate 

 of lime. There are also some dark opaque masses, of irregidar size 

 and shape, scattered in both matrix a3±d'tuT)es. 



That the tubes are to a certain extent hollow, is shown by the 

 great abundance of small bells of air which remain in them on 

 addition of turpentine. "While, on the other hand, that they are 

 not altogether hollow is certain, not merely from the small size of 

 the air bells, which remain distinct even when crowded thickly 

 together, are with difficulty expelled, and do not move about ; but 

 also, because transverse sections exhibit only some irregular perfora- 

 tions, while the cut ends of the tubes stain uniformly with carmine. 

 The large dilatations, above alluded to, however, are hollow in the 

 centre. 



When sections of the plates are calcined, the contents of the tubes 

 are burned entirely away, and only the matrix remains, having suffered 

 just sufficient change to render it less transparent, and caj^able 

 of readily imbibing the carmine die. 



The tubes have a somewhat irregularly nodulated outline, as if 

 composed of large cells placed end to end. Their contents have an 

 obscurely granular appearance, which becomes more distinct towards 

 the deep aspects of the plates, where one can even distinguish in 

 them bodies like small cells. In sections steeped in nitric acid, 

 which removes the mineral matter with effervescence, the granular 

 appearance of the contents of the tubes comes out much more 

 strongly, while the matrix and superficial spines appear structureless. 



In turpentine the matrix exliibits, at many places, a number of 

 irregularly matted fine black lines, somewhat resembling those which 

 may be seen in calcifications of fibrous tumours. They are found 

 most abundantly ami closely felted beneath the bases of the super- 

 ficial spines. 



"When the structure of the dermal plates, now described, is 

 compared with that of the general integumeut it becomes apparent, 

 I think, that the former structm*e is derived from the latter, by 



