﻿BEITISH FOSSIL CEPHALOPODA. 133 



Orthoceras subundulatum, Portlock, PL XI. figs. 8, 9, 10. 



1843. Orthoceras subundulatum, Portlock, ' Geol. Eep.' pi. 28, fig. 2, p. 373. 

 1852. „ „ M'Coy, 'Pal. Fossils,' p. 317 (part). 



1854. „ „ Salter in Murchison's ' Siluria,' Foss. gr. 62, fig. 3. 



Syn. 1843. Orthoceras incertom, Portlock, loc. cib., pi. 28, fig. 7, p. 374. 



1846. Creseis Sedgwickii, Forbes, ' Quart. Journ. Geol. Soc' -vol. i. p. 146, fig. 2. 



1866. Orthoceras socium, Barrande, ' Syst. Sil. de Boheme,' pi. 222, 372, &c. 



Query 1863. Orthoceras subundulatum, Haswell, 'Silurian Eocks of the Pentlands,' pi. 1, 

 figs. 10, 14. 



Type. — The specimen is flattened, so that the diameters are in the ratio of 3 to 1. 

 The rate of increase of the flattened shell is 1 in 8. The ornaments are upward 

 imbrications which have an undulating course, but no general obliquity ; they are 

 from 3 to 4 per line, but there are 3 or 4 smaller ones between each ; towards the 

 apex the smaller alone are present. There are still deeper lines at irregular dis- 

 tances, from 1^ to f of a line apart, which are not seen at the smaller end. 

 These have been taken for indications of septa, which they very much resemble, 

 but, as they are only seen on part of the shell, have an undulating outline parallel 

 to the smaller ornaments, and are not regularly spaced ; they are more probably 

 mere variations of the finer imbrications. If this be so, the septal characters are 

 not shown. The length is •§- inch and the greatest diameter -| inch. From the Bala 

 Series of Fermanagh. In the Museum of Practical Geology. 



General Description. — The section has been seen to be circular in one example 

 and nearly so in another. The rate of increase is very variable, not only on account 

 of the compression, but in individuals from the same locality. The average is that 

 of the type, but in some it is 1 in 6, in others, more rarely, 1 in 20. The ornaments 

 are characteristic, well-marked upward imbrications. These have an undulating 

 course, with very slight general obliquity ; their distance is variable on the same 

 specimens, but is seldom more than \ line. In the body -chamber there are, in one, 

 broad undulations marked by the greater and less distance of the ornaments in series. 

 One specimen shows a curious feature on one side, consisting of lines which undulate 

 in a contrary direction to the ordinary ornaments, and thus bound a set of diamond- 

 shaped transverse depressions ; others show more elongated depressions (fig. 10), but 

 all these may be due to the flattening of the shell. The body-chamber is at least as 

 long as its basal diameter, but the aperture has not been seen. The septa are direct 

 and distant ^ of the longer diameter of the examples showing them. The orna- 

 ments are seen not to be parallel to the septa, but to cut them in their undulations ; 

 about 3 or 4 ornaments cross one chamber. The last two or three septa are closer 

 than the rest. The siphuncle has not been certainly seen, but a small septal surface, 



