468 ; BRITISH PALASOZOIC FOSSILS. [Bracniopopa. 
hinge-line #4, width of front {;, depth between the two valves ji, length of visceral disk in entering valve =, 
width of ditto #%, width of flattened border to ditto ;. 
The character of the remarkable geniculation forming the narrow flattened border to the visceral disk, is 
so well shewn in Martin’s figures, that it is surprising that it has been overlooked by modern writers, being 
supposed probably to represent the thickness of stone between the two valves: an attentive examination will 
shew, however, that this is not the case, a portion of the shell smoother than the rest being bent at right angles 
to the preceding, and to the following parts of the valve, covering over that peculiar margin represented in 
Martin’s plate. This character seems to be only developed in the adult, or when the visceral disk of the enter- 
ing valve is seven or eight lines long, when the width of the border is rather more than a line. This internal 
character entirely separates the species from the P. semireticulata, with which the continental writers unite it ; 
from which it also differs in its small size, narrow elongate, and undulated front, and broadly-waving closer 
and smoother striae, and by the very few, aud nearly obsolete concentric wrinkles, which even on the casts 
of the entering valve produce no sensible interruption of the longitudinal strie. The irregular waving striz 
contrast strongly with the direct sulcation of P. sulcata, semireticulata, &c. 
Position and Locality——Not uncommon in the lower limestone of Derbyshire ; and middle limestone of 
Poolwash, Isle of Man; rare in the carboniferous limestone of Berwick on Tweed. 
Propucta MESOLOBA (P/iil.) 
Ref.—1d. id, Phill. Geol. t. 7. f. 12; de Koninck, Monog. Prod. t. 17. f. 2. 
Dese.—Subquadrate ; hinge-line slightly exceeding the width of the shell; receiving valve with the rostral 
portion for about seven lines from the beak moderately convex, and marked with small irregularly undulating, 
interrupted, rounded, concentric wrinkles (four in two lines about middle of disk); beak small, slightly projecting ; 
ears flattened, of moderate size, slightly acute, obscurely defined from the body of the shell; from the concen- 
trically wrinkled rostral portion, the front descends with an abrupt curve nearly at right angles to its plane, 
slightly flattened in front, and obscurely compressed on the sides from about three lines from the beak; a thick 
rounded mesial ridge, one line wide throughout, extends to the front margin, defined by a rather wide shallow 
depression on each side; spine-bases thick, large, conic, rather few, very irregularly distributed over the whole 
shell, four or five on the ridge. Surface smooth or with very fine undulating transverse striee of growth. Entering 
valve almost following the curvature of the receiving one, and similarly marked in all respects in intaglio, but 
the visceral disk rather more flattened, and the mesial furrow less sharply defined. Average width one inch six 
lines, length of visceral disk %, of smooth front =; (longer when old). 
The shell is extremely thin, and when partially decorticated, a very minute longitudinal lineation is seen 
with the lens. This species is so strongly marked by its narrow mesial ridge, that it cannot be confounded 
with any but the P. pralonga (Sow.) =P. Christiani (de Kon.), from which it is distinguished by the want of 
the longitudinal suleation of the surface. 
Position and Locality——Not uncommon in the dark carboniferous limestone of Kendal, Westmoreland ; 
not rare in the lower carboniferous limestone of Derbyshire; also in the red carboniferous grits of Breedon Hill, 
Leicestershire ; dark lower carboniferous limestone of Poolwash, Isle of Man. 
PRODUCTA PLICATILIS (So7-) 
Ref.—Id. id. Min. Con. t. 459. f. 2; de Koninck, Monog. Prod. t. 5. f. 6. 
Desc.—Hemiprismatic, transversely oblong ; hinge-line equalling ‘the width of the shell. Receiving valve 
having the visceral disk subquadrate or nearly semicircular, moderately convex in the middle, rather abruptly 
rounded to the large flattened ears, on the sides entirely covered up to the hinge-line with strong, narrow, 
rounded, slightly undulating concentric wrinkles (three to four in two lines in middle), crossed in parts of some 
specimens by fine regular thread-like striz (ten in two lines); beak very small, scarcely passing the hinge-line ; 
