STRUCTURE OF SHELL. 43 



which has been made by some authors, I do not think is 

 very accurate, as the cup in Lithotrya seems to have a 

 special relation to the boring habits of that genus. 



Structure of the individual compartments. — If the basal 

 margin of a compartment, for instance, of Balanus tintin- 

 nabulum, be examined, it appears 

 sufficiently complicated, being Fig. 9. 



composed of an outer and inner 

 lamina, separated by longitudi- 

 nal septa, which are denticulated 

 at their bases ; and the tubes 



x* 11. +1 „ 1„„ ™i-,, J™«1 Basal edge of wall of compartment 



formed by these longitudinal mBatmustintinnahduw, a, a, aa.ua 



aania ova nvnaeor] hv trnnc lamina; b, h, inner lamina; c, c, lon- 



Sepid aie ClOSSetl Oy liailb- gitodinal septa, uniting the inner and 



verse septa. On the other hand, %£***"*■ with theu ' ends denti ' 

 in some cases, as in the genera 



Chthamalus and Elminius, each compartment consists 

 of a simple shelly layer. These two extreme states gra- 

 duate into each other : we have, firstly, on the internal 

 surface, quite irregular points and ridges ; these become 

 regular, causing the internal surface to be longitudinally 

 ribbed ; then these ribs themselves become finely furrowed 

 on their sides and at their lower ends, producing sharp, 

 minute ridges, the ends of which I have called the den- 

 ticuli ; and, lastly, some of the denticuli on the adjoining 

 longitudinal septa become united into a solid layer, forming 

 the internal lamina of the wall. These denticuli do not 

 generally cover the whole surface of the longitudinal ribs, 

 but leave a portion near the outer lamina of the compart- 

 ment smooth. The denticulated ends of the longitudinal 

 septa project beyond the basal edge of both the outer and 

 inner laminae, and enter the mouths of the tubes (where 

 such occur) in the basis, and thus strengthen the shell. 

 The whole of the internal lamina generally is more or less 

 striated longitudinally, thus displaying its origin from the 

 union of the inner edges of the longitudinal septa. I need 

 only further remark that on the internal surface of the outer 

 lamina, between the main longitudinal septa, there are 

 generally (as in the woodcut) smaller longitudinal ridges, 

 which do not reach the inner lamina, and on this account 

 alone are not called septa 



