82 ORDOVICIAX AXD SILURIAN BELLEROPHOXTACEA. 



it, orbicular. Diameter of the spire 1 inch 8 lines, thickness 1 inch, longest 

 diameter of the aperture 3 inches, rather longer than wide. The last whorl before 

 it expands to form the large aperture, is twice as wide as long-. The edge of the 

 aperture embraces two-thirds of the discoid spire ; the front of it has no fissure, 

 although there is a ridge upon the whorl which indicates the existence of such a 

 fissure at an earlier period of growth. Two of our specimens show furrows 

 inside the mouth ; the one from the Lower Ludlow Rock is nearly smooth, but has 

 slight indications of them ; may not the former be impressions of the outer surface ? 

 Loc. Burrington, near Ludlow." 



It will be noticed that he does not mention the foramina, but there are distinct 

 traces of their presence in his type-specimen, though it is not as well preserved 

 in this region as could be wished. The presence of external revolving ridges 

 is also doubtful, as the shell is missing on the dorsum. 



Lindstrom 1 was doubtful if his T. longitudinal-is was identical with Sowerby's 

 species, and remarked that neither Sowerby nor McCoy make mention of the 

 presence of foramina in the dorsal line. McCoy's 2 description of the species was 

 based on the Girvan specimens from Mulloch Hill, which show the revolving lines 

 distinctly and are here regarded as a separate species referable to Phragmostoma 

 (see Phr. decipiens). 



R. B. Newton (pp. fit.) has given a full description of certain specimens in the 

 British Museum, from the Wenlock Beds of Dudley, which he describes as Tremato- 

 notus britannicus, but it does not seem possible to separate them from Sowerby's 

 B. dilatatus, though they are very much better preserved than the latter's types, 

 and show the foramina on the dorsum with great clearness as well as the ornamen- 

 tation of thick sinuous revolving lines. Newton states that there is no sign of an 

 apertural sinus and that the margin is entire, but the broad shallow open notch 

 figured by Lindstrom in his Tr. longitudinalis appears to exist. 



The specimen [27997] from the "Wenlock Shale of Dudley in the Jermyn 

 Street Museum, though distorted, may be referred to this species ; it shows well 

 the spiral lines and foramina as in Newton's figured specimens of Tr. britannicus, 

 but concentric stria? seem more developed round the reflected margins of the 

 mouth where the spiral lines have completely died out. 



The difference in the appearance of the exterior of these shells seems largely 

 due to the fact that the shell itself consists of more than one layer, the inner one 

 bearing the revolving lines, as Perner 3 has pointed out. The Bohemian species 

 Tr. beraunensis, Perner, 4 from Stage Ee2, which shows the structure distinctly, is 

 allied to our Tr. dilatatus. 



1 Lindstrom, op. cit.. p. 86, pi. iii, figs. 39, 40; pi. iv, figs. 1 — 7. 



McCoy, op. cit., p. 309. 

 3 Perner, op. cit., pp. 104, 105. 

 * Ibid., p. 114, pi. lxxxii, figs. 26—29; pi. lxxxvi, figs. 45, 46; text-figs. 80—82. 



