Professor E. J. Garwood — Rock-huilding Organisms. 267 



uniformly in Ortonella than in Miteheldeania, while the angle of 

 divergence of the branches is also much greater in Ortotiella, thus 

 giving rise to the marked difference in the spacing of the tubes in 

 the tvro genera. It also differs from Miteheldeania, as described by 

 Nicholson, in the absence of cross partitions to the tubes and the 

 imperforate character of the walls, and also in the general absence 

 of the liner series of tubes wliich Wethered and Nicholson regard 

 as an essential character of Miteheldeania (PI. XXI, Fig. 2). The 

 structures described by Nicholson as cross partitions in the tubes 

 are, however, very irregular and frequently entirely absent in the 

 Cumberland examples of M. gregaria, while the perforations in the 

 cell-walls mentioned by the same author are very obscure, and, as 

 I have already stated,' I have been unable to discover any true pores 

 in the Cumberland specimens of M.gregaria, even in those which I have 

 collected from Nicholson's type locality at Kershope Foot. It is 

 possible, therefore, that the large pores figured by Nicholson are in 

 reality the elbows of the undulating tubes intersected by the plane of 

 , the section. 



As regards the finer series of tubes in Miteheldeania (PL XXI, 

 Fig. 2), these appear to be typically present in examples of 

 M. Nicholsoni and in most of the larger specimens of 31. yregaria. 

 I have, however, frequently found them to be absent from small 

 examples of M. gregaria, the nodules in this case being entirely 

 made up of the coarser tubes. At the same time it is not an 

 uncommon occurrence to find small nodules entirely composed of 

 the finer tubes without any traces of the coarser forms, and it is open 

 to question whether the two sets of tubes necessarily belong to the 

 same organism. This doubt seems to be strengthened by the 

 occurrence, though rarely, of specimens of Ortonella in Westmor- 

 land with tufts of fine tubules associated with them, which are 

 indistinguishable from the finer tubes in M. gregaria. 



Other organisms to which Ortonella bears some resemblance are 

 Girvanella, Nich., and Sphccrocodium, Kothpl. It differs, however, 

 from these genera in the radial arrangement of its tubes, which in 

 Girvanella and Spharocodimn lie in concentric layers round the 

 nucleus. Again, though Rothpletz has described dichotomous 

 branching in Girvanella, this is certainly obscure in most British 

 examples, including Nicholson's genotype from Girvan. It appears, 

 however, from Dr. Ilothpletz's description, to occur in Sphcerocodium 

 Bornemanni. 



Systematic Position. — The genus Ortonella, as shown above, seems 

 to be most nearly allied to Miteheldeania and Sphcerocodium, and, as 

 both these forms appear referable to the calcareous algaj, Ortonella 

 may also be provisionally placed among the Thallophyta. As regards 

 its more definite systematic position, it seems to be most nearly 

 related to the Siphonese, but, as Professor Seward cautiously remarks 

 regarding Girvanella, Siphonema, and Spheerocoditwi, " It is wiser to 

 regard such tubular structures as closely allied organisms, which are 



' Rep. Brit. Assoc, Section C, 1913, and Geol. Mag., Dec. V, Vol. X, 

 pp. 440, 490, 545. 



