464 BRITISH PALALOZOIC FOSSILS. { Bracuropopa. 
now in the University collection, enables me to state positively that the distinctions relied on by M. de Verneuil 
in the Geology of Russia, and M. de Koninck in his Monograph of Productus for separating the P. latissima from 
the P. gigantea, do not really exist; several specimens in the collection shewing in one individual the gradual 
conoidal passage of the ear into the body of the shell (as in P. latissima) on the one side, and a narrow 
subeylindrical ear, projecting abruptly from the side of the defined gibbous body of the shell (as in the 
P. gigantea) on the other; the other characters mentioned as distinctive by them and the older authors, 
such as thinness of the shell, and few or no longitudinal folds, greater depression of the ears, &c., are characters 
of the young shell, and particularly of the entering valve ; further, both forms (contrary to what those authors 
suppose) occur together in abundance in the same bed in Northumberland. The careful proportional measure- 
ments which I have made of a great number of specimens, the extremes of which I have given above, prove in 
like manner what I could not otherwise have anticipated, that the P. crassa of Martin is only an extreme 
variety of the P. gigantea; as that acute observer himself thought probable, and as I have now traced the 
passage in the most gradual manner. Young specimens six lines long are rather more than twice as wide as 
long, only slightly convex, transversely elliptical, with obtusely elliptical ears, having coarse rounded sulei 
(about seven in three lines at four lines from the beak, where they first dichotomise) ; after this size the beak 
and ears, particularly of the entering valve, begin to inroll rapidly. TI believe the form called P. Edelburgensis 
by Phillips cannot be kept apart when many specimens are examined; for I have counted the same number of 
strie in a given spot on highly typical specimens of P. gigantea, which M. de Verneuil gives as distinctive 
for P. Edelburgensis ; the flattening of the ears given by him and Phillips is manifestly variable in the specimens 
before me, and spines certainly also occur on specimens otherwise agreeing with P. Hdelburgensis. The variety 
named P. Scotica seems certainly to be founded on young examples of the less transverse varieties, and to 
be distinguished by its spines, coarse irregular strie, and form of the muscular impressions, &c., from the 
true P. hemispherica ; which see. 
Position and Locality—Common in the black carboniferous limestone of Dalmellington, Dumfriesshire ; 
common in the grey lower carboniferous limestone of Derbyshire; very abundant in the dark limestone of 
Lowick, Northumberland ; common in the limestone of Corwen; black limestone of Craige, near Kilmarnock ; 
red limestone of Closeburn, Dumfriesshire ; in the limestone of Kendal; dark limestone of Ronalds-way, Isle 
of Man. 
PRODUCTA HEMISPH#RICA (Sov.) 
Ref. and Syn. = 1d. id. Sow. Min. Con. t. 328. < Producta aurita Phill. Geol. York. t. 7. f. 6. 
Desc.—Shell hemispherical, regular, margin even. Receiving valve evenly gibbous in the middle, and 
gradually and equally sloping towards the front and side margins. Entering valve nearly as concave as the 
receiving one is convex; hinge-line equal to the width of the shell, forming short, obtusely-rounded, convex 
ears, passing gradually into the margin, but rather abruptly defined from the body of the shell; the hinge-line 
often with a crowded pectinated row of short, slender, curved spines. Radiating strize close, obtuse, convex, 
regular, scarcely flexuous, separated by very slender impressed lines (averaging eight or nine, rarely six, in two 
lines, at one inch from the beak) nearly equal ; the young intercalated striae reaching the thickness of the others 
at about half an inch from their origin, smooth without spines; ears with small, short, irregularly transverse 
plicee, becoming quickly obsolete. Average width two inches six lines, in proportion to width, length of receiving 
valve 4, of entering =, depth 4, space between the valves ;5. 
In this species the entering valve is almost as concave as the receiving one is convex, so that there results 
between them an exceedingly small space for the animal; the P. personata (Sow.) is probably an internal cast 
of this species. The internal casi of receiving valve is very different from that of the P. gigantea, even when 
specimens of the same size are compared, most remarkably so in the proportion of the closely sulcated or 
anterior portion of the great muscular impressions, which in the present species are twice as wide as long, 
and separated by a space equal to about twice the height of that portion, while in P. gigantea the same 
closely striated anterior muscular impressions are at least as long as wide, and separated by a space only equal 
