522 BRITISH PALAZOZOIC FOSSILS. [GasrErRopopa. 
There is great variety in all the characters of this extremely variable fossil: the ridging of some specimens, 
is larger, and of many smaller, than the above; the larger the ridges the more regular are they in their distri- 
bution and size; some are more nearly orbicular, and some more oblique and angulated ; but these changes are so 
irrelative to each other, that it is obvious they all belong to one species, which, from the extreme thinness and 
flexibility of its substance, was liable to accidental variations from mechanical causes, in addition to those to 
which it was probably liable when living. Some of the specimens also shew, here and there, faint radiating 
puckerings, giving rise to the supposed species P. tuberculata (Sow.), which I am satisfied is but a slight 
variety. 
Position and Locality Extremely abundant in the black culm limestone, in some parts at Venn, near 
Barnstaple ; equally common in the black limestone and shales over the main limestone of Derbyshire; com- 
mon in the black limestone and shale of Poolwash, Isle of Man. 
Var. a. TUBERCULATA (Sow.) 
Ref. and Syn. = Posidonia tuberculata Sow. Geol. Trans. 2nd Series, Vol. V. t. 52. f. 5. 
Agrees in all respects with the ordinary types of P. Becheri, except in having a few obscure, radiating 
furrows from the beak, faintly tuberculating the concentric ridges; these are, however, so indistinct, irregular, 
and uncertain in their development, as scarcely to entitle the specimens having them to rank even as a mode- 
rately constant variety ; those specimens in which the radiations appear near the beak frequently want them on 
the posterior half of the length, so that one half of the specimen might be referred to P. Becheri, and the other 
half to P. tuberculata, if that form were to be admitted as a distinct species; other specimens again shew 
these radiations on the posterior portion, and not near the beak. 
Position and Locality.—Not uncommon with the plain P. Becheri in the black culm limestone near Barn- 
staple. 
PoOSIDONIA LATERALIS (,S07.) 
Ref.—Sow. Geol. Trans, 2nd Series, Vol. V. t. 52. f. 1. 
Dese.—Oblongo-elliptical, very much elongate obliquely ; beaks close to the anterior end, which is very 
short and semielliptically rounded ; dorsal margin very slightly convex, extremely thin; compressed, raised con- 
siderably above the level of the beaks; greatest width near the posterior end, which is semielliptically rounded, 
the curve passing gradually into that of the dorsal and ventral margins, the latter being gently convex or nearly 
straight in old specimens; sides moderately convex from the beaks, and marked with subregular, large, con- 
centric angular ridges; narrow and sharp on the middle and anterior part, broad and obtuse on the posterior 
portion, becoming abruptly obsolete near the dorsal margin, along which a narrow space is left smooth, or only 
marked with lines of growth, parallel with the dorsal edge ; sometimes a few obscure radiating furrows decus- 
sate the ridges on the posterior slope. Length of old specimens four inches, proportional length of anterior end 
im» Proportional width from beak to opposite ventral margin 4, width near posterior end *, greatest depth 
(about the middle) ;;;. At one inch from the beak, near the anterior ventral margin, six or seven ridges occupy 
_ three lines ; at the same distance from the beak on the posterior slope, there are but two in the same space in 
one strongly-marked specimen; the ridges retain nearly the same dimensions at all further distances, but at the 
extreme posterior end there are four ridges in the same space ; a second specimen, of equal dimensions and the 
same shape as above, has about seven ridges in three lines anywhere between the beak and ventral margin, and 
about three in about the same space on the middle of the posterior slope, or four in the same space nearer to the 
beak or to the posterior end. 
The few radiating furrows on the posterior portion of one of the specimens are analogous to those on the 
variety of P. Becheri, forming the so-called P. tuberculata of Sowerby. The long dorsal margin should by no 
means be confounded with a hinge-line, as it is marked with lines of growth parallel to its edge. The figures 
given by Professor Phillips (Pal. Foss. f. 74) would induce a belief that this may only be an extremely oblique, 
elongate, old variety of P. Becheri. The substance is excessively thin, and the surface is marked with obsolete 
minute lines of growth, parallel to the ridging. 
Position and Locality —Common in the black culm limestone of Venn. 
