ANIMALS. 131 



regular rounded imbricated laminae of growth, which are crossed with numerous fine 

 striae : w^ith a wdde, 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. Umbones rather strongly incurved. Fissure 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 Spirifera convohita ; 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. convohita ; 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, PI. 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 Trigo- 

 notreta speciosa, an analogous species, in which this structure is completely formed 

 in 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 PI. 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 in the compact limestone of Midderidge. It also occurs at Schmerbach, 

 Roepsen, Merzenberg, Konitz, Poessneck, Ronneburg, Gera, Seissla, Wohlsdorf, 

 Ilmenau and Noberg, in Germany. 



Trigonotreta undulata, /. de C. Sowerbg. PI. IX, figs. 1, 2, 3, 13, 14, 15, 16, 17. 



Spirifek undulattjs, J. de C. Sow. Mineral Concbology, vol. vi, p. 119, pi. .562, fig. 1. 



— — „ Genera of Shells, Plate of Spirifer, fig. 3. 



— — „ Sedgwick, Trans. Geol. Soc. London, 2d series, 



vol. iii, p. 119, 1829. 



— — „ De la Beche, Geol. Man., p. 384, 1831 ; Germ. 



Transl. p. 459, 1832; and 3d Eng. ed., p. 572, 

 1833. 



