48 



i 



frequently met with specimens in that state, but as yet hare never seen any side 

 anus clasping round extraneous matter. 



The COLUMN. (PL. i. and PL. n. fig. 24. and 26.) The place where the late 

 J. TOBIN'S, Esq. specimen broke off, was in a part of the column where its 

 joints had acquired a degree of maturity, and the fracture goes slantingly 

 through several of them, thereby demonstrating the small quantity of membrane 

 intervening between them, its tenacity, and also the thinness of the exterior 

 integument that surrounds the joints, which is now so dried up as to be al- 

 most imperceptible, being of a pale yellow, and intimately connected with 

 the calcareous laminar secreted matter forming them. As the column is not 

 broken in such a manner as to afford a correct idea of the adhering surface of 

 the joints, I must refer for this point to the fossil remains of this species occur- 

 ing in the lyas. The alimentary canal and its investing membrane are very 

 small. The shape of the column being pentangular, and more or less indented 

 between the five salient angles. (PL. n. fig. 24. and 26.) We trace on the in- 

 ferior and superior surface of each columnar joint five subovate petal-like 

 figures, marked by elevated ridges radiating round the margin of each of them, 

 the interior of each of these five petal-like figures forms an oblong, ovate, 

 smooth space, surrounded by the marginal radii, opening by a narrow groove 

 at the inner end, which is the most pointed, into the alimentary canal. On 

 joints not fully formed there is also a depression proceeding from the alimen- 

 tary canal, between the marginal radii surrounding the sides of the contiguous 

 petal-like division, to a smooth space beyond them, filling up the more or less 

 indented, and frequently contracted, intervals between them. If we consider 

 this construction in an early stale of growth, when the joints are very thin and 

 muscular, and the calcareous secretion is only commencing, and hence follow 

 the operation of nature, we may suppose that the calcareous deposit first 

 formed round the alimentary canal, in the space occupied by the five petal-like 

 divisions and their marginal radii, and thus continues to increase till the joint 

 has acquired a thickness consistent with the proportions and size of the animal. 

 The space between the five petal-like divisions continues muscular a consider- 

 able time longer, and begins only at a later period, and very gradually, to se- 

 crete calcareous matter to strengthen the deposit round the alimentary canal, 

 and to interpose thin layers to keep apart the petal-like divisions. The thin 

 muscular integument investing the alimentary canal, probably forms a sphinc- 

 ter at the junction of each two joints, dividing here into ten conspicuous por- 

 tions or muscular integuments, one extending to each of the spaces between 



