ANIMALS. 131 
regular rounded imbricated laminze of growth, which are crossed with numerous fine 
strize: with a wide, moderately deep, median furrow ; numerous rounded, occasionally 
dichotomous ribs on the lateral surfaces, gradually decreasing in size as they approach 
the sides; and a faint one in the median furrow: occasionally plicated parallel to 
the cardinal line. Umdones rather strongly incurved. /sswre open when young, and 
covered with a laminated deltidium in old specimens. Dental plates small, curving, 
and coalescing. 
In my ‘ Catalogue’ this species is identified with Phillips’s Spifera convoluta ; but 
having lately examined specimens of the latter shell in the British Museum, I am now 
satisfied that the identification is erroneous. Both species are obviously closely 
related to each other; but there are several small ribs in the median furrow in 
T. convoluta ; whereas in T. alata there is only one thus situated: and the lateral ribs 
run out from the cardinal region more obliquely in the former than in the latter. 
The dental plates in this species have an unusual form, being small, curving 
and coalescing at their upper part, so as to become arch-shaped, as represented 
in fig. 12, Pl. IX. The deltidium is also unusual in its structure, as it consists 
of strong arching lamellae. Specimens attain a tolerably large size before the 
deltidium is formed,—a peculiarity which distinguishes this shell from Zrigo- 
notreta speciosa, an analogous species, in which this structure is completely formed 
im much younger or smaller specimens. ‘The spiral processes have a close resemblance 
to those represented by Professor M‘Coy (vide Synopsis, p. 127, fig. 15), except that 
the free or projecting portion of their crura is longer. The crura of the spiral, 
represented in Pl. IX, fig. 8, are so invested with foreign mineral matter, that it is 
impossible to say whether they remain separated or become united through curving 
towards each other, as in the Jurassic Trigonotretas already noticed; appearances, 
however, do not oppose the latter being the case. The punctures differ widely from 
those characteristic of the preceding species, in being so small as to be only visible by 
a high magnifying power. 
Trigonotreta alata is not an uncommon species in the shell limestone of Humbleton 
Hill, and m the compact limestone of Midderidge. It also occurs at Schmerbach, 
Reepsen, Merzenberg, Konitz, Poessneck, Ronneburg, Gera, Seissla, Wohlsdorf, 
Iimenau and Noberg, in Germany. 
TRIGONOTRETA UNDULATA, J. de C. Sowerby. PI. IX, figs. 1, 2, 3, 13, 14, 15, L6galiee 
SPIRIFER UNDULATUS, J. de OC. Sow. Mineral Conchology, vol. vi, p. 119, pl. 562, fig. 1. 
= — 6 Genera of Shells, Plate of Spivifer, fig. 3. 
=e — Sedgwick, Trans. Geol. Soc. London, 2d series, 
vol. ii, p. 119, 1829. 
= — iS De la Beche, Geol. Man., p. 384, 1831; Germ. 
Transl. p. 459, 1832; and3d Eng. ed., p. 9572, 
1833. 
