Chap. IX ] 
LOWER SILURIAN MOLLUSKS. 
195 
though not found in our Lower, is now known to be present in our Upper 
Silurian *. 
The primeval forms allied to the genus Terebratula are now mostly referred 
to the genus Phynchonella t of Fischer. This genus is very much less frequent 
in the lower than in the upper division of the system. Some few forms, 
described in the 1 Silurian System/ chiefly belong to the Llandovery rocks, 
and will be noticed under that head. One or two plaited species are 
known to occur in the Caradoc beds of Snowdon, and in the rocks near St. 
David's, Pembrokeshire, some of which are of Llandeilo age. Similar species, 
too, are found in Tyrone, and have been described by Portlock. A few smooth 
species, like Phynchonella navicula, PI. XXII. f. 12, but not identical with it, 
are found in the Bala Limestone. Rh. (Athyris ?) depressa, PI. XXII. f. 17, and 
Rh. rotunda, f. 18, originally described from the Upper Silurian, are said by 
Prof. M/Coy to occur in true Lower Silurian strata. 
The genus Atrypa, so often quoted in palaeontological works, is now re- 
stricted, and includes only those shells which have calcareous spires inside, and 
beneath the beak or umbone a small area pierced by a round foramen. Thus 
limited, it contains only a few species ; and these are most abundant in all the 
Upper Silurian strata, and in the Llandovery rocks. Atrypa marginalis of Dal- 
man is the only one known with certainty to occur in the Caradoc Sandstone ; 
and it is found in many localities in Wales and Ireland. 
Of the next great class, the Lamellibranchiata, or ordinary Bivalve Shells, a 
few only had been discovered in the lower group of strata when my large work 
was published. They are now, however, ascertained to be far from rare in the 
more arenaceous parts of the Caradoc strata in England ; and they also occur in 
rocks of the same age in America. The genus Pterinea (Avicula, Sil. Syst.), so 
common, as will hereafter appear, in the upper division, has some representa- 
tives in the lower. Pt. pleuroptera, Conrad, and Pt. tenuistriata, JVPCoy 
(Chap. X. p. 228, Foss. 60. f. 5), are two species of this formation, quoted 
by M'Coy as ranging upwards to the Ludlow rock. It is probable that some 
other species of the genus have an equal range. 
There appear to be some species of true Aviculae ; and there are several ex- 
amples of that section (or rather of that distinct genus, Ambonychia, Hall) which 
has both valves gibbous and the anterior ear almost obsolete. These shells have 
been sometimes called Inoceramus, but have no affinity with that group. 
Ambonychia Triton, Foss. 39. £ 8, for example, is found at Llandeilo ; and 
Portlock describes Amb. transversa and Amb. trigona from the rich deposits of 
Tyrone. 
Modiolopsis, Hall, is a still more common genus in the Lower Silurian. M. 
orbicularis, PI. VII. f. 1, one of the few species published in my former work, is at 
present known only in the Caradoc Sandstone of Shropshire ; it was formerly de- 
scribed as an Avicula. M. modiolaris, Foss. 39. f. 3, and M. postlineata, f. 1, 
are frequent in the Caradoc rocks of North Wales, where M. mnata,M'Coy, and 
some others are also occasionally met with. M. expansa, f. 2, is a flattish spe- 
cies found in strata of the same age in Ireland ; and several large and rounded 
forms occur with it. The genus is closely allied to the common Modiola, or 
Mussel, of our coasts. 
Nucula-like shells with toothed hinges are not scarce ; but few of them have 
been yet described. It is well, however, to note that the small triangular spe- 
* Davidson's Monograph of the Brachiopoda, Palaeont, Sec. 1854, p. 136. 
t Some also fall into the genus Atrypa. 
