GENUS CHTHAMALUS. 451 



cimens, the whole basis was calcareous, being absolutely 

 continuous with the inner lamina of the parietes, showing 

 that the latter had grown flatly inwards all round, and 

 had then become confluent in the middle, so that there was 

 no longer any basal membrane; excepting, no doubt, that 

 which had existed in the younger stage, and which would 

 be preserved in a functionless condition between the surface 

 of attachment and the inflected parietes. 



When an opercular valve or compartment is dissolved in 

 acid, layers of tissue are left, and these are seen to be pe- 

 netrated by tubuli, which enter at the punctures before 

 mentioned on the inside of the shell : these tubuli often 

 stand in groups of three or four together ; they are about 



5 th of an inch in diameter. Besides these irregularly 



lOOOO' 



scattered tubuli, there are in the opercular valves of C. 

 antennatus, innumerable smaller parallel tubuli, running 

 to the external investing membrane. 



Structure of the Badii and Aim. — The radii, when de- 

 veloped, are always rather narrow. Their recipient furrows 

 are generally nearly as broad as the radii themselves. Their 

 edges are either quite smooth, as in C. antennatus ; or very 

 finely crenated; or, as in C. dentatus and Hembeli (PL 18, 

 fig. 3 a, 5 a), so strongly crenated as to make the suture, 

 both externally and internally, toothed : in these two species, 

 the radii are ribbed in transverse lines parallel to the basis, 

 each rib corresponding with one of the projecting and in- 

 terlocking teeth on the sutures. In C. intertextus, and 

 much less plainly in most specimens of C. scabrosus (PI. 19, 

 fig. 1 a, 2 a), we have a structure in appearance very different, 

 for the radii here consist of several very oblique plates, (i. e. 

 nearly parallel to the parietes) on both sides of the sutures, 

 which are interfolded or locked together: I believe that this 

 structure is a mere modification of that in C. dentatus and 

 Hembeli, the transverse ridges on the radii of those species 

 being here developed into oblique plates. We shall hereafter 

 meet with a similar structure in the genus Verruca ; to which 

 genus, until meeting with these two species of Chthamalus, 

 I had thought that the interfolding sutures had been confined. 

 The alee have their edges generally finely crenated : during 

 diametric growth (when such takes place), they are rarely 



