730 DH. W. B. CAEPENTER ON THE STRUCTCEE, PHTSIOLOGT, AND 



the apices of these plates incline to one another, so as to form a five-side.1 pyr.'mdal 

 cover to the calyx (Plate XXXIX. fig. 2), the plates will close together not merely 

 nt their apices and lateral margins, but also at the upper part of their internal surfaces. 

 There is also a broad depression near the base of each plate, so that its lower margin is 

 somewhat everted. The anal plate («), which is intercalated between two of the first 

 radials, has a tolerably regular circular shape ; but it consists of only a single cribriform 

 film, and has no definite border. 



62. Concurrently with the advance in the development of the Calyx, the Stem under- 

 -goes-an increase both in the number and in the length of its component segments ; and 

 while it also increases to some extent in its diameter, its solidity is still more augmented 

 by the endogenous growth of its calcareous skeleton. The cribriform plate on which it 

 rests augments both in diameter and in thickness, absorbing into itself (as it were) nearly 

 the whole of the organic substance of the basal disk. Its typical form may be considered 

 as circular; but its margin is usually more or less deeply divided into lobes (Plate XXXIX. 

 fig. 1). Its diameter is usually about "OlS inch. In its centre is a deep depression that 

 lodges the end of the lowest joint of the stem. — It will be remembered that the deve- 

 lopment of each of the original segments of the Stem was shown by Professor Wtville 

 Thomson to commence by the solidification of a ring, which occupies what will afterwards 

 become the middle of its length ; and that from each of the two surfaces of this ring a 

 hollow cylinder of calcareous trellis-work gradually extends itself. The length of each 

 of these original joints is augmented by new calcareous deposit at its extremities, which 

 finally become compactly rounded off and well defined, so that the apposed surfaces of 

 two segments are clearly marked out from each other, instead of having their irregu- 

 larities commingled, as in the earlier period of their formation. The diameter of each 

 segment increases by noAV calcareous deposit on its cylindrical surface, bringing up its 

 whole length to the size of the first-formed median ring, and finally giving to its 

 extremities a slight excess beyond this. At the same time the solidity of each segment 

 is increased by an inward extension of the calcareous trellis-work, which progressively 

 fills up what was at first a hollow cylinder. This internal solidification, however, 

 goes on more slowly than the completion of the external form and dimensions of the 

 segments ; for these may present their mature aspect, or nearly so, whilst possessing so 

 little substance that their shape is altered, by the drying up of the soft sarcode-axis 

 of their interior, to that represented in Plate XLI. fig. 4. 



63. While the original segments, which are stated by Professor Wyville Thomson' to 

 be eight in number, are thus advancing towards completion, new segments are being 

 developed in the interval between the highest of these and the base of the calyx. 

 None of my specimens in the stage now described present fewer than ttoelve segments ; 

 so that four at least must have been thus interpolated. That the upper end of the 

 Stem is the part at which the new segments originate, is very plainly indicated by several 

 considerations. It is there that, during the period in which the stem is continuing to 

 increase, we always find the most rudimentary segments; and we may trace, as we 



