Chap. X-.] 
UPPEK SILUEIAN GASTEKOPODA. 
231 
larger proportion in species is still on the side of the Upper Silurian rocks ; 
whilst the individuals in them are unquestionably more numerous. 
The suggestive remark of Prof. Phillips must here be noticed, that the families 
to which the Silurian Lamellibranchiata generally belong are those which occur 
at the junction, so to speak, of the Monomyarian and Dimyarian groups. Myti- 
loid, Nuculoid, and Aviculoid shells, therefore, with few additions, were the re- 
presentatives of this order in the Silurian seas. 
Gasteropodous Mollusks, or Univalve Shells, are spread throughout the Upper 
Silurian, but seldom form a conspicuous feature. The Wenlock limestone, how- 
ever, is neither poor in species nor individuals, though several are yet unpub- 
lished. Among them Euomphalus occupies a marked place, four species espe- 
cially swarming in certain localities, viz. Eu. discors, Sow., PI. XXIV. f. 12, Eu. 
rugosus, Sow., f. 13, Eu. funatus, Sow., PI. XXV. f. 3, and Eu. alatus, His., f. 4. 
A species also very like Eu. centrifugus, Wahl., not unfrequently occurs with 
them. Euomphalus sculptus, Sow., f. 2, which appears to be only a variety of 
Eu. funatus, Sow., is found as frequently in the Llandovery as in Wenlock 
rocks. Eu. alatus, His., f. 4, is chiefly a Wenlock Shale species, and occurs of 
the largest size in the calcareous slates of the Dingle promontory in Ireland. It 
is there associated with several other species, particularly the large Eu. lautus, 
M'Coy, Eu. carinatus, Sow., PI. XXIV. f. 11, and Trochonema (Eu.) tricinctus, 
M'Coy, the second of which is most characteristic of the Middle Ludlow or 
Aymestry rock. Of all these species, Eu. funatus, Sow., is by far the most com- 
mon, and has the greatest vertical range. Its concentric operculum is often met 
with, and helps to show the near relation of the genus to Delphinula, as above 
stated (p. 196, Note). 
Eunema cirrhosa, PI. XXIV. f. 10, often occurs in the Wenlock Shale ; and 
Platychisma Williamsii, PI. XXXIV. f. 13, together with some allied species, is 
frequent in the Upper Ludlow. The Tilestone is crowded with Platychisma 
helicites, PI. XXXIV. f. 12, 13, a shell which used to be known as a ' Trochus,' 
and looks like a land-snail in outward form. Trochus ? cselatulus, M'Coy, is a 
rare species from the Woolhope Limestone ; and Cyclonema undifera, M'Goy, is 
a small species of the Aymestry rock. 
The Turritellse, ' Sil. Syst.,' of the uppermost Ludlow rocks have been already 
referred to the genus Holopella, p. 196,— H. obsoleta, Sow., PI. XXXIV. f. 11, 
H. gregaria, Sow., f. 10 «, and H. conica, Sow., f. 10, being very frequent fossils 
in this stratum in Westmoreland, Shropshire, and South Wales. Prof. M'Coy 
has distinguished another small species, H. gracilior, from the Wenlock Shale 
of Llangollen. Loxonema sinuosa, Sow., PI. XXIV. f. 3, and L. elegans, M'Coy, 
are chiefly Ludlow rock species. The latter is a fine shell, two inches long, and 
is frequent both in the Wenlock and Ludlow shales. 
Spiral Shells with notched apertures, Pleurotomaria and Murchisonia, are com- 
mon. Murchisonia corallii, Sow., PI. XXIV. f. 7, M. articulata, Sow., f. 2, and 
M. torquata, M'Coy, are slender turreted forms in the Ludlow rocks. M. cingu- 
lata, His., three inches long, from Aymestry, is found also in Gothland. M. 
Lloydii, Sow., PI. XXIV. f. 5, abounds in the Middle and Lower Ludlow, and is 
frequent in Wenlock Limestone. Murchisonia balteata, Phillips (Mem. Geol. 
Surv. Vol. ii. pt. 1. pi. 15), is another interesting Wenlock form. Pleurotomaria 
undata, PI. XXIV. f. 6, is from the Lower Ludlow, where some other large spe- 
cies, yet unpublished, are also found. Acroculia Haliotis, PI. XXIV. f. 9, and 
A. prototypa, f. 8, are exceedingly abundant, the first especially, in the Wenlock 
Limestone, and also in the Upper Silurian of Bohemia. These mollusks seem 
