104 Frederick Clufpinan : 



Occurrence. — Abundant in the Silurian (Melbournian) of the 

 Yarra Improvement Works, South Yarra. Coll. by F. P. Spry 

 and the writer. 



Affinities. — T. crassituba seems most nearly allied with T. 

 squamosa, Phillips, ^ but differs in the thicker and consequently 

 more rigid tube. The English specimens of T. squamosa were 

 found at two different horizons in the Silurian, viz., the 

 Woolhope Limestone and the Upper Ludlow beds. 2 



Remarks. — ^Many less perfect specimens of Tracliyderma have 

 been found at South Yarra, which, although apparently refer- 

 able to 2^. crassituha, show only a stain around the core, as if 

 the wall of the tube had been dissolved away (see Fig. 4). In others 

 the tube is still existent, but nearly of the same substance as the 

 surrounding shale, although of harder texture, so that a frac- 

 ture in the right direction reveals its tubular nature. The cores 

 or infillings of the burrows show a tendency to crack into short 

 joints, owing to differential shrinkage. It is evident that the 

 departure from the circular section in the contour of the tube 

 is due to pressure of the superposed layers of hardened mud, 

 for the nearer the burrow to the horizontal position, the more 

 compressed does it tend to become. This points to the inevit- 

 able conclusion that the tube, however thick it may have been, 

 was always more or less membranous and compressible. 



Tpachyderma cf. squamosa, Phillips. 

 (PI. XXVII., Fig. 5). 



2\ squamosa, Phillips, 1848, Mem. Geol. Surv. Gt. Brit., vol. 

 ii., pt. 1, p. 332, pi. iv. figs. 3, 4, 



Remarks. — An example of Trachy derma, which does not show 

 the thick tube of the Melbournian specimens, occurs at another 

 horizon, the Yeringian. In this specimen there are some traces 

 of the plaits and fine striae of Phillips' species, so that we may 

 provisionally refer it to that form. The tube passes at an 

 oblique angle into the shale bed, and is elliptical in section. 



1 Mem. Geol. Surv. Gt. Brit., vol. ii., pt. i., 1848, p. o32, i)l. i\ ., fi^'s. o, 4. 



2 Fossils of the British Islands, litlicridse, H., vol. i., Palaeozoic, 1S88, \>. 37. 



