54 



In the 2 entire undivided specimens which I have delineated tig. 3 & 6, the propor- 

 tions of the joints of the stem were tolerably uniform. The first knee is (and this seems to 

 he the rule for all specimens) situated close down to the funnel-shaped enlarged part of the 

 root. After this comes a very long calcareous joint, then 2 shorter joints, of which the first 

 is shortest; then again a long joint nearly of the same length as the first. The other joints 

 (4 or 5 in number) seem to be all about of the same length. Also in the specimen deline- 

 ated fig. 4 the proportion of the joints appears to have been originally similar, supposing 

 that the small second joint, which follows on the first side-branch, has properly belonged to 

 the next following. Likewise a similar proportion will be observed in the piece of stem de- 

 lineated fig. 22, presuming that the small first joint originally formed part of the next follow- 

 ing. Finally if we assume that in the 2 single specimens (fig. 3 & 6) the 2 — and in the 

 branched specimen (fig. 4) the 3 joints following the first had originally formed a single joint, 

 it will appear that the stem consists of a series of about equally long joints. It seems to 

 me very probable that in reality, as the colony grows up, such a division of the calcareous 

 joints may take place into several smaller joints connected with each other by horny knees, 

 whereby the joints become gradually more numerous, and shorter in .proportion to their thick- 

 ness. In all cases I have usually found that the joints in younger colonies are not only 

 relatively, but absolutely longer than in the older colonies. Tliis is quite different from what, 

 according to Steenstrup ', may be observed in the genus Isis, where on the contrary there 

 occurs during the growth a fusion of joints originally separated, so as to form coherent cal- 

 careous parts, particularly in the main stem, which on that account often seems to be to a 

 great extent uujointed. In this coral such a fusion never seems to take place; the stem is 

 always distinctly jointed right down to the root; and the joints seem just here to be shortest, 

 as the division of the originally single joints seems just here preferably to take place. This 

 division of the calcareous joints does not occur equally everywhere, but begins earlier in cer- 

 tain places, which just seems to account for the frequently great difference in the length of 

 the joints in one and the same colony. Probably therefore the calcareous joints in very large 

 colonies, in which this division is further advanced, are on the whole proportionally of much 

 less elongated form than in younger colonies, even if there may not be so small a difference 

 between the calcareous and horny joints as in the other species of the same genus. 



As regards the branches, their axis is just like the main stem completely divided 

 into calcareous joints connected with each other by horny knees, about of the same thickness 

 as the joints of the main axis. They are as already stated, always single or unbranched, and 

 usually take their origin from the horny joints (see fig. 11), a feature which is precisely fun- 

 damental in the genus Mopsea, and whereby this genus is distinguished from the allied genus 

 Isis, where the branches, on the contrary, take their origin from the calcareous joints them- 

 selves. This rule is however not without exception. I have now and then found calcareous 

 joints on which a distinct side-process had foi'med itself (fig. 15) which seems to be the be- 



' Om Slie>;ten Isis os; de under Isis liippmis Linn, sunimeiiljliiiiileile Arler |ig. 7. 



