Bracuioropa.] LOWER PALAOZOIC MOLLUSCA. 201 
Silurian type. The general form slightly recalls that of the H. Wilsoni, but there are much fewer ribs, 
and they are not divided by a medial sulcus on the deflected front. 
Position and Locality—V ery rare in the Upper Ludlow rock of Burton and Brockton, near Wenlock. 
HEMITHYRIS DEPRESSA (Sow. Sp.) 
Ref. and Syn.=Atrypa depressa Sow. Sil. Syst. t. 18. f. 6. 
Sp. Ch.—Transyersely broad—oval ; receiving valve with a small, prominent, pointed beak, beneath which 
is a distinct, narrow, triangular, opening ; rostral portion gently convex, sides flattened, middle deeply depressed 
towards the margin by a wide triangular sinus, disappearing considerably in front of the beak; entering 
valve deeper than the receiving one, profile strongly arched, greatest depth at rather more than one-third from 
the beak, obtusely subcarinate along the middle, rising to a wide, low, convex mesial ridge towards the front 
margin, sides sloping rapidly to the margin, with a slight convexity; lateral margins sharp, slightly sigmoid, 
front margin raised into a deep, wide, rotundato-quadrate sinus; surface smooth, with regular, obtuse, 
strong, concentric striz towards the margin. Average width three lines, proportional length *%, depth ;%. 
It is only in one specimen out of fourteen examined, that I see the slightest trace of the three 
obscure furrows along the middle mentioned by Mr Sowerby. The depth varies considerably, being com- 
monly greater than represented by Sowerby’s figure. 
Position and Locality — Abundant in the Bala limestone of Brynbedwog Quarry, near Bala, Merionethshire. 
HemitHyris DipYMA (Dal. Sp.) 
Ref. and Syn.=Terebratula didyma Dal. Vet. Acad. Handl. t. 6. f. 7. 
Sp. Ch.—Pentagonal ; apical angle 87°; valves very gibbous, smooth, each with a narrow mesial sulcus, 
reaching two-thirds to the beak, and notching the front margin, which is slightly cornute from the con- 
cavity of the antero-lateral margins; length six lines, proportional width 4, depth of receiving valve alone 
3, length of entering valve {; shell fibrous. 
This species is distinguished by its much greater gibbosity from the H. Upsilon of the lower strata, 
and by its width from the H. canalis. 
Position and Locality.— Wenlock limestone, Ledbury, Herefordshire. 
HemitHyris pDioponta (Dal. Sp.) 
Ref. and Syn.= Terebratula id + T. bidentata Dal. t. 6. f. 4.=Ter. bidentata Sow. Sil. Syst. t. 12. f. 13 a. 
= Ter. decemplicata Sow. Sil. Syst. t. 21. f. 17. 
Sp. Ch.—Subrhomboidal, apical angle 115°; sides of both valves slightly convex, moderately deep ; 
mesial ridge and hollow very large, and sharply defined, forming an abruptly defined angular sinus in the 
front margin, nearly as high as wide, bidentate by two strong angular ridges on the entering valye, and 
one in the middle of the sinus of the receiving valve; lateral ribs rather smaller than the mesial ones, five 
or six in number, all continued simply to the beak; surface crossed by fine transverse lines of growth. 
Average width (of small specimen) five and half lines, proportional length ;°;, length of entering valve &. 
Position and Locality —Not common in the sandstone of Pwllheli, Caernarvonshire, and sandy schists 
of Yspatty Evan, N. Wales; limestone of Balmae Shore, Kirkeudbright. 
HEMITHYRIS HEMISPHARICA (Sow. Sp.) 
Ref. and Syn.= Atrypa hemispherica Sow. Sil. Syst. t. 20. f. 7. 
Sp. Ch.—Suborbicular, small valve flat, with a slightly deflected margin and inconspicuous beak; large 
valve evenly gibbous, subhemisph-rrical, with minute prominent beak; apical angle about 140°; hinge-line 
nearly as long as the shell is wide, cardinal angles obtuse; both valves radiated with about seventeen 
subequal, rounded, or obtusely angular ridges, separated by concave spaces resembling them in size and 
[wase. 11.] Do 
