294 Prof. Nicholson and R. Etheridge, jun. — 



itself permanently to the columns of crinoids. The subject has also 

 been entered on at some length by one of the present writers, in a 

 paper lately read before the Natural History Society of Glasgow. 

 The Geological Collection of the British Museum contains a number 

 of specimens of Carboniferous Crinoid stems singularly contorted 

 and gnarled by the growth of Monilopora ( Cladochonus) crassa and 

 the subsequent efforts of the Crinoid to envelope its parasite. These 

 are contained in the collections of the late Messrs. Gilbertson and 

 Rofe. In almost every case the corallum is more or less entirely en- 

 closed in the substance of the crinoid stem, and those with the coral- 

 lites projecting for any distance from its periphery are exceedingly un- 

 common. This has been explained by Mr. Rofe on the supposition, 

 that those not enveloped escaped simply because of the death of the 

 crinoid before complete enlargement of the column had taken place. 

 The usual external appearance presented by the combined crinoid 

 stem and its encircling parasite is that of a swollen, or contorted stem, 

 with a ring of circular apertures, sometimes with projecting lips, at 

 other times on a level with the surface of the stem, or even with the 

 substance of the latter closing over them. This last is much the most 

 common condition, but occasionally we meet with two or more rings 

 one under the other, and instances are before us in which successive 

 growths have taken place so rapidly as almost to obliterate all 

 trace of the original crinoid stem. Of specimens with the calices 

 partially free, Mr. Rofe has figured a good example ; of those with 

 the mouths only showing in a single circle round the crinoid stem 

 we give an example, and another where two circlets are preserved, 

 and a third illustrating the confused condition produced by con- 

 tinued growth. In only one specimen have we been enabled to 

 detect any trace of septa — and then only in the form of indistinct 

 fiutings in the interior of one of the calices. It should, however, 

 be remarked that the specimens which have come under our obser- 

 vation have not been in the best state of preservation for the reten- 

 tion of delicate septal markings. 



As regards the mode of growth of Monilopora crassa, the usual 

 method adopted by the coral consists in the encircling of a crinoid 

 stem by a single ring of corallites, the connecting stolons being 

 applied directly to the surface of the crinoid stem, and the corallites 

 thrown off from this at intervals and projecting outwards at a more 

 or less oblique angle to the stem to which they are attached ; while 

 occasionally they project nearly at right angles. On this simple 

 structure successive growths take place until a more or less confused 

 aggregation of corallites results. 



The very curious microscopic structure of the corallum in M. 

 crassa was, so far as we know, first pointed out by Mr. Eofe (Geol. 

 Mag. 1869, Vol. VI. p. 352, Figs. 4, 4a), though we are of opinion, 

 as previously stated, that his conclusion as to its real significance 

 was founded upon a misapprehension. He pointed out that there 

 existed in the corallum of this form a singular reticulated structure, 

 which he regarded as showing that the coral was " tabulate," and 

 he figured this structure in the paper just referred to. Our own 



