Fig. 
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ba Ge 
PLATE IX. 
Trigonotreta undulata, J. de C. Sowerby. Testiferous specimen, natural size, show 
ing area (a), and fissure or deltidium (4). Midderidge. 
(This beautiful specimen, which is the original of the figure in the 
‘Mineral Conchology’ (Pl. 562, fig. 1), has been kindly lent me by 
its owner, Mr. J. de C. Sowerby, for illustration. I have carefully 
removed some of the investing matrix, thereby showing certain parts 
more distinctly than in the figure just cited.) 
= — Large valve of the same specimen. 
— — Lateral aspect of the same. 
Trigonotreta alata, Schlothem. Cast, natural size, showing area, fissure, and small 
valve. Humbleton Hill. 
— a Cast (natural size) of the large valve. Humbleton Hill. 
— — Cast (natural size) of the small valve, showing impressions 
of the anterior division of the valvular muscles (a) ; 
posterior division of the same (4); and vascular 
system (c). Humbleton Hill. 
— aa Gutta-percha cast of the last specimen, showing area (q) ; 
excavated (? cardinal) muscular impressions (4); and 
origin of the crura of the spirals (c). 
— — Cast, natural size, showing one of the spiral coils. Hum- 
bleton Hill. 
— — Testiferous specimen, natural size, showing lamine of 
growth. Midderidge. 
— — Cast of a young specimen, rather enlarged, showing ribs 
parallel to the hinge-line. Humbleton Hill. 
= = Cast, natural size. Humbleton Hill. 
= ao Gutta-percha cast of the medial region of the area, showmg 
the fissure closed inwardly by an arch-shaped plate. 
(The lower cone-like portion bounded by the dark line, should not have 
been represented as it is foreign matter). 
. Trigonotreta undulata, J. de C. Sow. Specimen, natural size, with the shell partially 
removed. Humbleton Hill. 
— — Cast, natural size. Humbleton Hill. 
-— = Testiferous specimen, natural size. Tunstall Hill. 
— a Young specimen, rather enlarged. ‘Tunstall Hull. 
— a Portion of the shell, highly magnified, showing the laminz 
to consist of capillary fibres. Tynemouth Cliff. 
Trigonotreta Permiana, King. Cast, natural size. Humbleton Hill. 
— — Cast (natural size) of large valve. Humbleton Hill. 
— — Cast, natural size. Humbleton Hill. 
— ne Cast (natural size) of large valve. Humbleton Hill. 
= = Small valve of the last specimen. 
—- « — Cast, twice the natural size. Humbleton Hill. 
== a Large valve of last specimen. 
