228 SILURIA. [Chap. X. 
Ludlow or Aymestry rock ; and L. cornea, PI. XXXIV. f. 2, abounds in the 
Tilestone of the Upper Ludlow. They are both well-marked species, and, with ' 
their congener L. minima, Sow., and the small but characteristic L. lata, Sow., 
L. striata, Sow., and L. Symondsi, Salter, of the Lower Ludlow rock, are all the 
species of the genus known in the Upper Silurian of Britain. The long-oval 
species, called after my esteemed friend the Rev. W. S. Symonds, of Pendock, 
ranges from the Upper Llandovery to the Ludlow rocks, and is a common 
species at Malvern, and at Buildwas in Shropshire. 
The Lamellibranchiate Bivalves, though numerous in species, have not yet 
been fully described. They consist chiefly, as Prof. Phillips has pointed out *, j 
of one or two closely related families, which are represented by the living 
forms Mytilus, Area or Nucula, and Avicula. When strictly defined, the more j 
modern genera Pullastra, Mya, Cypricardia, and Cardium, to which several of 
these forms were referred in the original Silurian work, are now found not to have [ 
existed in these early geological times. 
Forms of the genus Pterinea (Avicula, Sil. Syst.) abound. Pt. retroflexa, 
Wahl., PI. XXIII. f. 17, a species subject to great variation, has already (pp. 99, j 
121 &c.) been quoted from various strata. Pt. Sowerbyi, M'Coy, f. 15, charac- • 
racterizes the Aymestry limestone and Ludlow rocks, whilst Pt. lineatula, Sow., { 
f. 16, is very frequent near Ludlow. Besides these common species, Prof. M'Coy ' 
has enumerated several others — namely, Pt. Boydii, Pt. demissa, Pt. pleuroptera, 
and Pt. subfalcata all of Conrad, and Pt. tenuistriata, M'Coy, Foss. 60. f. 5. The j 
last three are, it is believed, also found in Lower Silurian rocks in Wales and I 
Westmoreland. Others are upper Silurian forms only ; such as Pt. hians, M'Coy, 
an Aymestry rock fossil, and Pt. asperula, M-Coy, f. 4, a fossil from the Wenlock 
shale at Builth. A common Wenlock species, doubtfully referable to this genus, 
is Pt. planulata, Conrad, Foss. 60. f. 6. Many species, however, of this age, par- 
ticularly those from Pembrokeshire and Ireland, are, as yet, imperfectly known. 
The district of the Dingle in Ireland, for example, is rich in fossils of this group. 
Of these, Pt. orbicularis, M'Coy, is one of the largest and most plentiful. Pt. 
posidoniaeformis and Pt. fimbriata are described by-the same author ; and there 
are other species, with several fine varieties of the common Pt. retroflexa, Wahl., 
in that prolific locality. 
Fossils (60). Upper Silurian Lamellibranchiata. 
1. Pleurorhynchus sequicostatus, 
Phill. 2, 3. Avicula Danbyi, M'Coy. 
4. Pterinea asperula, M'Coy. 5. 
Pter. tenuistriata, M'C. 6. Pter.? 
planulata, Conrad. 
Other large species, more resembling Avicula, or Aviculopecten, such as A. 
Danbyi, M'Coy, Foss. 60. f. 2, 3, and A. ampliata, Phill., occur in the Upper 
Ludlow rock ; whilst Pterinea (Avicula) mira, Barr., a beautifully reticulated 
* Mem. Gool. Surv. vol. ii. pt. 1. p. 2fi4. 
