TREMATONOTUS AYMESTRIENSIS. 79 



is also a small V-shaped dorsal sinus. The presence of a sharp keel on the 

 dorsum near the mouth is indicated in the cast. Perhaps this shell is allied to S. 

 richmondense, Ulrich, 1 but it is too imperfectly known for an accurate comparison. 



Dimensions. — Height of mouth, c. 55 mm. ; width of mouth, c. 40 mm. 



Horizon. — Sholeshook Limestone, Upper Ordovician. 



Locality. — Sholeshook, Haverfordwest. 



Sub-Group Terebrodoksata. 



Genus TREMATONOTUS, Hall. 



Generic Characters. — Shell enrolled like Salpingostoma. Mouth suddenly 

 expanded, trumpet-shaped ; border entire. Umbilicus wide, open, exposing inner 

 whorls. Dorsum of each whorl with median row of closely -placed numerous 

 rounded openings. Sculpture of shell consisting of revolving ridges and transverse 

 undulating lamella?. Shell composed of three layers with a fourth external one 

 developed in old individuals. 



Without discussing here the validity of the separation of Phragmosfoma from 

 Trematonotus, into which question Perner 2 has gone at some length, we use the 

 generic name here in the same manner as he does, which is more restricted than 

 that adopted by Ulrich and Scofield. 3 The type is T. alpheus, Hall [? = Bucania 

 chicagoensis, McChesney], from the Niagara Group, and the genus seems almost 

 limited to the Upper Silurian. 



BeHerophon dilatatus, Sowerby, 4, is a well-known British example, and the 

 specimens in the British Museum described by Mr. R. B. Newton as Trematonotus 

 britannicus 6 are now referred to this species (see below). 



1. Trematonotus aymestriensis (Sowerby). Plate XII, figs. 7, 8. 



1839. BeHerophon aymestriensis, Sowerby, in Murchison's Silurian System, p. 616, pi. vi, fig. 12. 

 1848. BeHerophon aymestriensis, Sowerby, Ferussac and D'Orbigny, Hist. Nat. Ceph., vol. i, p. 216, 



pi. viii, fig. 15. 

 ? 1860. BeHerophon aymestriensis, Sowerby, Eichwald, Leth. Boss., vol. i, Anc. Per., p. 1084. 

 1885. BeHerophon dilatatus, Sowerby, Etheridge (pars), Brit. Pal. Foss., p. 119. 

 ? 1884. Tremanottis comjn-essus, Lindstrom, Silur. Gastrop. Pterop. Gotland, p. 87, pi. iv, figs. 8 — 12. 



Specific Characters. — Shell discoidal, of 4 — 5 whorls in contact but scarcely 



1 Ulrich and Scofield, op. cit., p. 903, pi. Ixvii, figs. 39, 40. 



2 Perner, op. cit., pp. 104, 121. 



3 Ulrich and Scofield, op. cit., p. 851. 



* Murchison, ' Silur. Syst.,' p. 627, pi. xii, figs. 23, 24. 

 5 Newton, ' Geol. Mag.' [3], vol. ix (1892), p. 337, pi. ix. 



