53 



middle, and shew regular concentric lines indicating the different calcareous strata which 

 have been deposited one outside of the other. As already stated, their length is rather un- 

 equal; as in the same colony there are often inserted single much shorter joints between the 

 longer ones (see fig. 4. 22 &c.). These shorter joints seem however to be restricted only to 

 the sterile part of the stem, while the joints in the polyp-bearing part and in the' branches 

 are more of one length. The horny joints or knees are everywhere of the same length, par- 

 ticularly short, scarcely more than half as long as they are wide; in fresh specimens about 

 of the same width as the calcareous joints, and like these striped longitudinally (see fig. 13). 

 The proportion of length between the calcareous and the horny joints, is thus in the present 

 species rather different from the other Isidinae; the calcareous joints being usually 15 20 

 times, and in younger stems (fig. 12), often 3040 times longer than the horny joints, while 

 the difference in length between both in the other Isidinae is far less. On more closely exa- 

 mining the mud drawn up in the dredge, there were found as above mentioned, besides root- 

 pieces and root-blades, numerous loose calcareous joints, and less frequently more coherent 

 pieces of stem (fig. 11. 12. 13. 14) of various thickness, and evidently belonging to colonies 

 of very unequal size. One of these calcareous joints, which I have delineated of the natural 

 size, fig. 16, was of quite unusual thickness, namely 5V 2 Mm. in the middle, but proportio- 

 nally very short, namely only 10 Mm. and had undoubtedly belonged to the sterile part of the 

 stem of an usually large specimen. Some few other joints, very short in proportion to their 

 thickness, were also found; but most were of the elongated form characteristic of the species, 

 for which reason I must consider such short joints, as those exceptionally inserted between 

 the longer joints, in conformity with what may be seen in the 2 younger colonies delineated 

 fig. 4 & 22. It will be seen that the thicker joints, derived consequently from larger colonies, 

 are on the whole proportionally shorter than the joints of younger colonies, which will imme- 

 diately strike the eye, on comparing the 3 pieces of stem all represented of the natural size 

 in fig. 12. 13 & 14. I will further, for better illustration of the unequal proportion between 

 thickness and length, add the measurements of a number of loose calcareous joints belonging 

 to colonies of various sizes. 



.V thick. 



long. 



