230 
SILUKIA. 
[Chap. X. 
easily recognized by the deep furrows on its valves. G. triangulata, Foss. 61, f. 2, 
is a typical Tilestone fossil, and G. cingulata, f. 1, as well as G. extrasulcata, 
Salter, are found with it, both in South Wales and Westmoreland. The last- 
named is equally common in other Upper Silurian strata (Wenlock), at Dudley, j 
Usk, and near Llandeilo, and is also found in Gothland, Sweden, and Norway. 
It appears to have flourished best on a sandy sea-bottom — a condition apparently j 
most favourable to the Lamellibranchiata in general. 
Cardiola interrupta, PI. XXIII. f. 12, is one of the most abundant Bivalves in 
the Wenlock and Ludlow Shales (see p. 127). On the Continent it is also found 
in Upper Silurian strata, but does not extend its range upward into the Devonian j 
rocks. Cardiola fibrosa, f. 11, is a Lower Ludlow fossil, and C. striata (Cardium, s 
* Sil. Syst.'), f. 13, is also to be referred to the same genus. 
The shells related to Nucula and Area, in which the hinge-line is beset with I 
close- ranged teeth, are very numerous in individuals, though of few species in , 
the Upper Silurian. Some of them may be true Nuculae, or even belong to the 
genus Leda, Forbes ; others will fall (see p. 196) into the genus Ctenodonta *, a 1 
name proposed in 1851 for those Palaeozoic Nuculae with an external ligament. 
Ctenodonta Anglica, d'Orb., PL XXIII. f. 10, Area ? primitiva, PhilL, Cteno- 
donta Edmondieeformis, M'Coy, and Ctenodonta subsequalis, PL X. f. 7, are 
Upper Ludlow shells, though most of them had an earlier origin, whilst Cteno- 
donta sulcata, His. (Mem. Geol. Surv. vol. ii. p. 269), ranges throughout the 
Upper Silurian. The genus Cucullella, distinguished by a strong internal ridge, 
contains several Ludlow species. C. antiqua, PL XXXIV. f. 16, C. Cawdori, 
f. 3, and especially C. ovata, f. 17, are common shells in the Uppermost Ludlow 
(Tilestones, ' Sil. Syst.'). C. coarctata, Phillips, is found in great plenty in the I 
Ludlow rocks of Pembrokeshire, and occurs also in the Wenlock Shale. The \ 
genus Cleidophorus, which has no teeth in the hinge, is otherwise much like 
Cucullella. CI. planulatus, Conrad, is found, according to M'Coy, in the Wen- 
lock Shale. 
Lyrodesma (Actinodonta) cuneata, PhilL, seems to connect these Area-like 
shells with the Mytilidse or Mussel family. Lyrodesma is found by hundreds in 
the shales of Marloes Bay — not, however, in the Upper Silurian, as formerly 
supposed (' Memoirs of the Geol. Survey,' vol. ii.), but in the Llandovery rocks. 
Lastly, as a representative of the Cardiaceae, we have one small species of 
Pleurorhynchus, a genus which, as we have before seen, p. 196, commenced ex- 
istence in the Lower Silurian. PL aequicostatus, Phillips, Foss. 60. f. 1, is a 
miniature example of it, found at Wenlock and Woolhope. In North America, 
larger species of this genus occur in Upper Silurian rocks. 
Lamellibranchiate Shells are far more frequently met with in the Upper Si- 
lurian strata than in the Lower — a fact indicated long ago by the organic remains 
figured in my former work, and confirmed by subsequent observations. Out of 
forty-five species quoted in Prof. Phillips's memoir t, published nine years after 
the 1 Silurian System,' five or six only are found in the Caradoc Sandstone and 
Llandeilo Flags. Prof. M'Coy, in his Descr. Pal. Foss. of the Woodwardian 
Museum, enumerates twenty-three species as belonging to the lower group, 
while he describes fifty from the Upper Silurian, as many as twelve being com- 
mon to both divisions. Portlock, indeed, described many Lower Silurian forms 
of this group from Ireland ; but an inspection of the Museum of the Geological 
Survey, and of numerous private collections, will convince any one that the 
* Kep. Brit. Assoc. 1851, Trans. Sect. p. 63. Descriptions of New Palaeozoic Fossils, Albany, 
Tellinomya of Hall proves to be the same genus, 1857, p. 141. 
though the name is not applicable. See his t Mem. Geol. Surv. vol. ii. pt. 2, 1848. 
