Chap. IX.] 
FOSSILS OF THE LLANDOVERY ROCKS. 
209 
the base of the series near Llandovery. The same species, PI. Rugeri, Salter *, 
also occurs in the immediately underlying Caradoc formation. When this family 
of Echinoderms appears in Upper Silurian strata, the species (and, indeed, most 
of the genera) are quite distinct from those of the Lower Silurian rocks. 
The Brachiopods are, as throughout the Silurian system, the most abundant 
fossils ; and, as might be expected, the species which range from the lower to 
the upper division are the most common. The genera Orthis, Leptaena, and 
Strophomena are still prevalent, and many of these are the same species which 
have been quoted as Lower Silurian types. Discina is seldom met with. Rhyn- 
chonella, Atrypa, and Spirifer, which are rare in Lower Silurian rocks, become 
plentiful. 
The Pentameri (including Stricklandinise) are, however, the characteristic 
fossils which impart to this zone its peculiar and distinct facies. No less than 
five species, whether smooth or only slightly ribbed, occur ; and of these, P. lens, 
Sow., and P. oblongus, Sow., are the best-known and the most widely spread. 
The typical species, P. oblongus, is easily distinguished from the others, P. lens 
and P. liratus, Sow., by the great length of the mesial septum, which in these 
latter is quite a short appendage to the V-shaped chamber *. The two longi- 
tudinal plates, also, which divide the upper valve, are peculiar to this species ; 
whilst in P. lens they are very short, and in P. liratus are reduced to a pair of 
processes which pass inwards, but do not show upon the surface of the cast 
(see Foss. 15. f . 1 & 3, p. 90) ; these last two species are referred by Mr. Billings 
to his new genus Stricklandinia Pentamerus globosus, Sow., is a rare species, 
easily recognized by its globular shape ; and P. undatus, Sow., is small and broad, 
with a wide central fold, and very short dividing plates. (See Plate VIII.) 
The distribution of these species is as follows : — In the Lower Llandovery 
rocks of South Wales, P. undatus, Sow., and P. (Stricklandinia) lens, Sow., are 
everywhere found, — P. globosus, Sow., and P. oblongus, Sow., being rare. In 
the upper series, whether in South Wales, Shropshire, or in the South of Scot- 
land (as at Saugh Hill near Girvan), P. oblongus is the prevailing fossil, occur- 
ring often in great banks to the exclusion of all other fossils, but generally 
accompanied by the species just named. At May Hill and the Malverns, Strick- 
landinia lens is by far the more common species ; St. lirata accompanies both 
these in the higher beds, and is the only one of the five species which ranges 
into the base of the Wenlock formation. 
Of Terebratuloid shells, two or three are typical of the formation. The cha- 
racteristic species of the Lower Llandovery rocks is the smooth shell Meristella 
angustifrons, M'Coy, Foss. 49. f. 2. It was first described from the sandstones 
of Mulloch, Ayrshire, and has since been detected near Builth, and in the 
lower beds at Llandovery. Meristella crassa, Sow., PL II. f. 7, is one of the com- 
monest species in the same beds, and apparently confined to them. M. furcata, 
Sow., PL IX. f. 12, one of the same group, is an Upper Llandovery species. 
Rhynchonella decemplicata, Sow., f. 15, is a characteristic shell of the Upper 
Llandovery rocks, and abounds through the Malvern and May Hill districts, and 
round the flanks of the Longmynd. It is never found in the Lower Llandovery 
beds, but occurs in the Denbighshire sandstones, associated with Upper Silurian 
* Mem. Geol. Surv. vol. iii. p. 288. the true Upper Silurian rocks. For a description 
t Pentamerus undatus, a most abundant species, of the structure of Pentamerus, consult Mr. 
is considered by M'Coy to be identical with P. Davidson's Monograph, Palaeontographical So- 
linguifer of the Woolhope and Wenlock lime- ciety, vol. i. p. 97 ; and for excellent descriptions 
stones. The great point, however, on which I of the species, see Prof. M'Coy in the Synopsis of 
relied in formerly classing these rocks as Lower the Woodwardian Museum. The P. microcamerus 
Silurian, was the invariable absence of their type of this author appears not to be distinct from P. 
shell P. oblongus, and its usual associates, from all lens. 
P 
