848 Transactions of the Society. 



It seems then to be quite certain that there is not the 

 distinction between the outer and inner portion of Cidarid spines 

 which Prof. Agassiz appears to have drawn. 



Another view taken with regard to the ostracum is that its 

 presence has the effect of terminating the growth of the spine ; the 

 evidence that I have to adduce will, I think, lead us to see that this 

 supposition is not well-founded. I do not know whether Dr. 

 Carpenter would express himself now exactly as he did in 1847, but 

 there is no doubt that what he said then has had a very consider- 

 able influence in determining the ideas and statements of succeed- 

 ing writers. In his well-known Keport to the British Association 

 on the microscopic structure of shells he says (p. 125), "This 

 much, however, seems certain, that whatever additions these spines 

 may receive in length they cannot be augmented in diameter, this 

 being fixed in the first instance by the production of the solid cal- 

 careous cylinder which forms the exterior of the spine." 



The observations which I now proceed to describe seem hardly 

 to support the views so clearly expressed by Dr. Carpenter, and 

 suggest rather the idea that the ostracum grows with the rest of 

 the spine, and retains the protoplasmic ground-substance which is 

 found in the rest of the tissue. 



1. As it is clearly impossible to make a transverse section 

 of a spine, and afterwards allow it to continue to grow, one has 

 had to be content with comparing a spine of fair size with one 

 that was a good deal larger, and was taken from a larger 

 specimen ; this last point is one of some importance, as the difierent 

 spines of one interambulacral area may vary considerably in length 

 in one and the same individual. The larger spine that I took was 

 one to the size and form of which the smaller might have been 

 fairly expected to grow had not its possessor fallen a victim to a 

 collector. 



The tips of two such spines of Oidaris metularia measured, 

 in transverse section, 1 mm. and 2 * 6 mm., while the crust, at its 

 thickest, was ' 227 mm. in the smaller, and • 4 mm. in the larger 

 specimen. The basal parts of two spines of the same species were 

 1'3 and 3 mm. in thickness; the crusts respectively '17 and 

 • 3 mm. Three spines of Phyllacanthus imperialis, in which the 

 transverse section was taken at the middle of their length, measured 

 respectively 3, 4, and 4 • 75 mm., and had a crust of • 3, * 35, and 

 •5 mm. thick. 



It is clear that, on the theory of determinate growth, as Mr. 

 Stewart has called it, the increase in the size of the spine ought to 



tage of similarity of nomenclature can be pleaded in defence of a course which is, 

 really, extremely inconvenient, and, so far as nomenclature is concerned, dis- 

 t^bing, if not revolutionary. 



