204 BRITISH PALAXOZOIC FOSSILS. {Bracuropopa. 
Position and Locality.— Not uncommon in the schistose limestone of Craig Head, near Girvan, 
Ayrshire. 
Explanation of Figures —P1. 1. L. fig. 5. Profile, natural size, from the limestone of Craig Head; 5 a, 
Ditto, receiving valve. 
HEMITHYRIS NAVICULA (Sow. Sp.) 
Ref. and Syn.= Terebratula navicula Sow. Sil. Syst. t. 5. f. 17. 
Sp. Ch.—Ovate, widest at one-third from the beak, front gradually narrowing, obtuse; receiving valve 
with a semicircular profile, middle obtusely subcarinate, sides steeply inclined to the margin; front margin 
forming a flattened arch towards the large valve; lateral margins broadly arched towards the small valve ; 
beak small, incurved; apical angle 85°; entering valve slightly convex at the sides about the middle of its 
length, with a medial depression widening from the beak to the front ; surface smooth, or with traces of faint 
radiating striae. Cast shewing a very deep V-shaped depression on the most convex part of the receiving 
valve below the beak towards which it points, and defining the muscular impressions ; nearer the beak are the 
usual pair of diverging dental lamellie ; a thick mesial septum extends from the beak nearly half ?the length of 
the entering valve. Length five lines, proportional width =, length of entering valve =, depth of both 
valves =. 
The peculiar chevron mark on the cast of the great valve is very remarkable, and makes the cast very 
easily recognisable ; it forms the anterior boundary apparently of two great, prominent, muscular impressions, 
which are marked by radiating sulci from the beak. 
Position and Locality—This well-marked fossil, so abundant at the base of the Upper Ludlow rock, 
abounds in the strata following ; sandstone of May Hill, Gloucestershire ; Welchpool, Montgomeryshire ; grey 
micaceous grits at Dinas Bran, Llangollen; schists of Mynydd y Gaer near Ruthin, (S. side); schists of 
Cwm Craig Ddu, Builth, Radnorshire ; olive schists of Erew Gill Fach, Builth; Cowan Head, Kendal, West- 
moreland; Potter’s Fell, Kendal, Westmoreland, 
HeMITHYRIS NUCULA (Sow. Sp.) 
Ref. and Syn. = Terebratula nucula Sow, Sil. Syst. t. 5. f. 20 = Terebratula pulchra Sow. Sil. Syst. t. 5. f. 21. 
= Atrypa semisulcata (Dal.) Salt. Quart. Geol. Journ. Vol. I. and Geol. Surv. Vol. II. 
Sp. Ch.—Broad—ovate, subpentagonal; beak moderate, pointed, apical angle 105°; receiving valve less 
tumid than the opposite one, gently convex at the sides and near the beak; becoming depressed from half 
its length into a gradually widening mesial hollow towards the margin, which is raised thereby into a deep 
rotundato-quadrate sinus, a third wider than high: entering valve evenly tumid, with a shallow medial depression 
near the beak, margin deflected, mesial ridge only marked near the margin ; surface radiated with about sixteen 
to twenty strong, smooth, angular, simple ridges, all continuing to the beak, about four (three in the sinus) in 
the middle somewhat larger than the rest towards the front, and forming strong prominent teeth to the sinus; 
the six to eight lateral ridges abruptly curved with the margin in front (most usually seven). Width six and 
half lines, proportional length =, length of entering valve =, depth of both valves 2. Casts shew two very 
short subparallel dental lamellze, one each side of the beak in the receiving valve ; the opposite valve has a strong 
medial septum, extending nearly half way from the beak to the margin, and a small ovate impression close to 
the beak on each side for the origin of the apophyses; tissue fibrous; foramen moderate; surface smooth, 
with a few longitudinal lines at the edge. 
Varies slightly in depth, and in the prominence of the mesial ridges at the margin. The limestone 
specimens are larger than those in the grits, and more obtuse. As there seems to be no such species as semi- 
sulcata described by Dalman, I have used Sowerby’s old name. 
Having carefully examined a large number of specimens, I think with Professor Bronn, that the 7. pulchra 
and 7’. nucula of Sowerby should be united, the very trifling difference in form, and number of lateral plaits 
