DESCRIPTIVE REMARKS. XXi 
rocks. O.calligramma, fig. 5,a common shell throughout the Lower Silu- 
rian, most characteristic of Caradoc or Bala strata, and Orthis striatula, fig. 
1, are figured, not for their abundance, but to complete the group of Llan- 
deilo Fossils. The Lingulee selected from amongst the half dozen or more 
species of this genus occurring in this Formation is Lingulaattenuata, fig. 
1, a, small shell, originally described and figured in the ‘Silurian Sys- 
tem;’ and LZ. plumbea, fig. 2, a more orbicular form, referred to Obolella 
by Mr. Salter, with respect to which Mr. Davidson, in his monograph on 
British Silurian Brachiopoda, observes that ‘‘it is not perfectly certain 
that any of our hitherto discovered British species agree with Mr. 
Billings’ genus, although two or three have been referred to it.”’ 
The very small orbicular Brachiopod, Siphonotreta micula, fig. 3, 
first described by Professor M‘Coy, is remarkable for its gregarious 
habits, as shown by the profusion in which it occurs; one hundred of 
these little shells having been counted on a fragment of shale four or 
five inches square from Pen Cerrig, Builth. 
The ordinary Bivalves, or Lamellibranchiata, are but few in number 
in these ancient deposits, as it is here only we observe the first cer- 
tain evidence of their appearance. Examples of these are given on Plate 
vill., figs. 7, 8, and 9, under the names of Palgarca, Redonia, and 
Ribieria. The Univalve or Gasteropod shells were likewise, but feebly, 
represented in this Formation, in which they also first make their ap- 
pearance. Huomphalus, fig. 10, is a genus of the Turbinide, very fre- 
quent in Paleozoic strata, ranging from these Llandeilo beds to the 
Trias ; Ophileta, fig. 11, being apparently a closely allied shell. 
To the Nucleobranchiata, which include the Heteropoda, shells, the 
animals of which swim at the surface, instead of creeping on the bed of 
the sea, Bellerophon, fig. 12, a convoluted shell, with a notched aperture, 
resembling somewhat that of the recent Atlanta, is[believed to have be- 
longed; a genus ranging from this formation through the entire series 
of Paleozoic strata. This class of the} Mollusca was developed to a 
much greater extent, anciently, than at the present day, particularly 
during the Silurian epoch. The Maclurea, fig. 13, a large discoidal shell 
resembling Euomphalus, with a solid operculum, fig. 14, corresponding 
with a species named M. Logani, which abounds in the Chazy limestone 
of the United States and Canada, is doubtfully included with Bellerophon 
in the family Atlantide, as a Nucleobranch shell.* It is also alluded 
to as probably belonging to Pteropoda, although the occurrence of this 
group in the older rocks is very obscure.+ 
The Pteropoda, a group of small animals, which appear in incredible 
numbers in the tropical seas, are, it is thought, represented Paleeontolo- 
gically by much larger forms, such as Zheca, species of which are 
figured on Plate v., fig. 9; and Plate viii, fig. 15; and Conularva, 
Plate v., fig. 10. 
The highest and most important class of Mollusca, that of the Ce- 
* Manual of the Mollusca, by S. P. Woodward, p. 202 (Weale’s Series). 
+ Paleontology, by Prof, R. Owen (1860), p. 72. 
