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



every direction. The tubes vary slightly in size, the lumina having 

 on an average a diameter of about 32 ju, and individual tubes show 

 a nearly uniform diameter throughout. The tubes are typically 

 straight or so slightly undulating that in thin slices passing 

 approximately through the centre of the nodule many of these tubes 

 remain in the plane of the section for considerable portions of their 

 length (Fig. 2). 



The tubes are circular in cross-section, completely separated, and 

 often widely spaced. They show marked dichotomous branching 

 (Figs. 3, 4), the angle of divergence of the branches being usually 

 about 40°, and there appears to be a tendency for this brandling to 

 take place in several neighbouring tubes at about the same distance 

 from the centre of the nodule. The tubes appear to be completely 

 devoid of transverse partitions, and no trace of perforations has been 

 observed in their walls. They are now filled partly with crystalline 

 calcite and partly with fine dark marly sediment, while the spaces 

 between the tubes are occupied by similar materials. The concentric 

 appearance of the nodules seems to be due parth'^ to the increased 

 proximity of the tubes where bifurcation occurs and partly to the 

 denser character of the matrix, which has remained more completely 

 entangled where the tubes are in closer approximation. 



Genotype. — Or totiella f areata, sp. nov. (PI. XX, Figs. 1-4.) All 

 the examples so far met with in Westmorland belong apparently 

 to one species, which, on account of its marked dichotomous 

 branching, we may appropriately speak of as Orto?iella ftircata. 



Horizon. — In the algal layer at the base of the Seminula gregaria 

 sub-zone, Athgrts glabristria zone ; Lower Carboniferous, Westmorland 

 and Lancashire. 



Localities. — Orton ; Ravenstonedale ; Shap ; Eskrigg Wood, near 

 Summerlands — in Westmorland; and near Low Meathop in Lancashire 

 also, sparingly, at about the same horizon immediately below the 

 Fell Sandstone in I^orth Cumberland and North- West Northumberland. 

 I have also met with a few examples, referable to this species but 

 with somewhat finer tubes, in the Modiola beds of Dr. Vaughan's 

 classification near the base of the Lower Carboniferous rocks in the 

 Avon Gorge. 



Resemhlances atid Differences. — In general appearance under the 

 microscope, this organism most closely resembles the genus Mitchel- 

 deania described by Mr. Wethered from the Lower Limestone Shales 

 of the Forest of Dean,^ a fuller account of which was subsequently 

 given by the late Professor Nicholson in 1888,- from the Lower 

 Carboniferous rocks of Kershope Foot on the Scottish Border. 



Our genus resembles Mitcheldeania gregaria, Nich., in the radiating 

 mode of arrani>ement of its tubes and in the fact that it exhibits 

 dichotomous branching. It differs from it, however, in the smuUer 

 size of the tubes, their straight and more uniform character, and 

 their much wider spacing. Again, though both genera exhibit 

 dichotomous branching this takes place much more frequently and 



^ Geol. Mag., Dec. Ill, Vol. Ill, p. 535, PL XIV, Fig. 6, 1886, and Proc. 

 Cotteswold Nat. Club, vol. ix, p. 77, 1886. 



- Geol. Mag., Dec. Ill, Vol. V, pp. 16-19, Figs. 1, 2, 1888. 



