BracuHtiopopa. | UPPER PALAZXOZOIC MOLLUSCA. 419 
six lines long semicircular or subtrigonal, twice as wide as long, depressed, with a very large, acutely angular, 
mesial ridge, and four or five large obtusely angular simple ridges on each side; lateral margins nearly hori- 
zontal, an acutely angular but slightly raised sinus in the middle of the front ; receiving valve moderately 
convex ; greatest depth in the middle, almost regularly arched from beak to front margin; about five thick 
obtusely angular simple ridges on each side of the middle, the two middle ones bounding the mesial sinus 
largest ; a narrower, strongly marked ridge in the centre of the mesial hollow, sharply defined from the apex 
of the beak, and more prominent than the rest towards the margin; beyond this size the lateral margins 
do not increase much in length, but by thickening their edges render the narrow sides cylindrical, from the 
centre of which the great mesial ridge of each valve forms a large, compressed, narrow, beak-like prolon- 
gation, elliptical in the profile; hinge-line as wide as the shell; cardinal area moderately wide, nearly parallel- 
sided; beaks remote, that of the receiving valve large, much incurved. Young specimen nine lines wide, 
proportional length of receiving valve =, entering valve 74, cardinal area ;, depth of both valves 7; adult 
length of receiving valve about one inch, length of entering valve nine lines, projection of front beyond the 
lateral margins five lines, depth of both valves in the middle seven and half lines, depth in middle of side five 
lines, width of cardinal area two lines, width of mesial sinus at junction of lateral margins four lines. 
This species when seen in front resembles the Roman letter T, from the long compressed extension of the 
central ridge of each valve at right angles to the cylindrical sides, and when viewed laterally, the resemblance to 
the beak of the little auk A/ca Alle has suggested the specific name. The young might be confounded with the 
S. triangularis of Martin, particularly as figured by Sowerby (Min. Con. t. 562, and de Koninck, t. 15), but is 
easily distinguished by the much smaller number of the lateral ridges, and the strong angular ridge in the centre 
of the mesial sinus of the receiving valve extending, with sharp definition, quite to the apew of the beak, and 
at even six lines long much exceeding all the others in prominence. In the adult state, or at and after an inch 
long, its appearance is so peculiar that it can be confounded with no other species, from the compressed beak- 
like extension of the front. 
Position and Locality —Rather rare in the carboniferous limestone of Derbyshire. 
Explanation of Figures—P\. 3. D. fig. 27, receiving valve of immature specimen, in which the mesial 
prolongation is not fully developed, the cylindrical sides both broken off; natural size; fig. 27 @, ditto profile of 
both valves. 
SPIRIFERA OVALIS (Phill.) Pl. 3. D. fig. 23. 
Ref. and Syn. = 1d. id. Phill. Geol. York. Vol. II. t. 10. f. 5.+ 8. (Brachythyris) hemispherica (M°Coy) Synop. 
Carb. Foss. Trel. t. 19. f. 10. S. rotundatus and pinguis de Kon. Anim. Foss. Bel. t. 15. f 4. (not of Sow.) 
Desc.—V ery broad-oval, width very slightly more or less than the length of receiving valve; length of 
hinge-line much less than half the width of the shell, scarcely interrupting the orbicular outline of the entering 
valve ; lateral margins nearly straight, abruptly raised in front into an obtusely-rounded sinus ; entering valve 
with a rather prominent tumid beak ; mesial ridge broad, depressed, rounded, very strongly defined by two 
deep sulci, either smooth, or sometimes very obscurely divided towards the margin by two faint obsolete sulci; 
ten or twelve thick, rounded, or very obtusely angular, usually simple ribs on each side. Receiving valve conoidal 
or subhemispherical, very gibbous at one fourth of the length from the beak, from whence it arches gradually to 
the front margin, and with a short but more rapid curve to the beak, which is narrow, small, moderately 
incurved, and much elevated by the triangular cardinal area, the height of which is considerably more than two- 
fifths of its width; mesial hollow obsolete in all the rostral portion, becoming moderately concave as it 
approaches the front sinus, and there usually marked with one or two obsolete, broad, longitudinal ridges. 
Width of var. ovalis one inch four lines, proportional length of receiving valve slightly more; length of 
43 
entering valve ;;, length of hinge-line ==, height of cardinal area =, depth of entering valve ==, depth of 
1009 
40 
receiving valve =, width of mesial ridge at front sinus ;;, height of sinus 3. Width of var. hemispherica one 
100 * 
inch five lines; proportional length of receiving valve 3, of entering valve %, depth of entering valve +4, of 
100? 
45 
receiving valve =, width of mesial ridge at front sinus =, height of sinus > 
luv? 100° 
