422 BRITISH PALAZOZOIC FOSSILS. [Bracuiopopa. 
cardinal angles smooth; cardinal area moderately defined, triangular, about two-thirds wider than high. Sur- 
face smooth or marked with inconspicuous concentric lines of growth; shell tissue finely fibrous under a strong 
90 
lens. Width of average-sized specimen seven lines, proportional length of receiving valve ;\,, length of entering 
valve **,, length of hinge-line ;7,, height of area about ;;,, depth of entering valve =, of receiving valve =, 
length of very large specimen ten and half lines, proportional width {,, depth of both valves 5 
100° 
Position and Locality —Not uncommon in the carboniferous limestone of Derbyshire. 
SPIRIFERA STRIATA (Mart. Sp.) 
Ref. and Syn. = Anomites striatus Martin, Pet. Derb. t. 23 ; = Spirifer id. Sow. Min. Con. t. 270. + 8. attenu- 
atus Sow. id. t. 493. f. 3, 4, 52+ S. clathrata M°Coy, Syn. Carb. Foss. Irel. t. 19. f. 9. 
Desc.—General form varying from subrhomboidal to semicircular, usually depressed ; cardinal angles acute, 
but varying from slightly so, to attenuated spines ; hinge-line as wide as the shell; cardinal area wide, concave, 
with subparallel sides. Entering valve subsemicircular, moderately convex, with a rather wide, very obtusely 
angular mesial ridge, varying greatly in prominence, and sometimes defined on each side by a deep sulcus, 
which is by no means constant; most strongly defined towards the margin of large specimens, where it is raised 
by the very deep, oblong, or semielliptical sinus, which raises the front margin nearly at right angles to the 
plane of the lateral edges, and gives a sigmoidal profile to the middle of this valve. Receiving valve moderately 
convex, flattened on the sides; beak moderate, slightly projecting ; a deep, wide mesial hollow extends from its 
apex to the sinus in the front margin, to fill which the middle of the receiving valve in old specimens is pro- 
duced into a long tongue-shaped lobe ; profile gradually and regularly arched from the beak to the front margin 
in old specimens, but the posterior third most abruptly arched in small specimens. The entering valve at one 
inch from the beak commonly exhibits eight or ten mesial narrow, subequal, rugged ridges, produced by the di- or 
trichotomy of a pair of lateral ridges, and the fourfold division of a primary one from the beak ; at the same length 
on each side are about twenty-five equal and similar ridges to the mesial ones, those eight or ten near the 
eardinal angles being commonly simple, those nearer the middle di- or tri-chotomising once or twice in that 
distance from the beak (averaging seven to ten in the space of half an inch) ; from this point to the margin the 
ribs increase in number by irregular forking at various distances, but do not vary materially in size, the above 
number of ridges in half an inch being found all over the surface of large specimens, the lateral ones remaining 
simple, but the sulci marking the divisions between the primary branching ridges remaining considerably deeper 
than the sulci separating the branches, so that a more or less distinct fasciculation is produced ; surface when 
perfectly preserved shewing under the lens a very minute reticulation, composed of equal, transverse, obtuse 
striz (distinct from the lines of growth), crossed by straight, equal, obtuse, longitudinal strize, straighter and 
much coarser than the fibrous tissue of the shell. Average width three inches nine lines, proportional length 
85 55 10 
of receiving valve = to =, length of entering valve >, width of cardinal area =, width of mesial sinus at 
front +, height of sinus ;; to =, depth of both valves ;;; to 7, or in a very long gibbous variety (resembling 
M. Mosquensis, but without the long dental lamellie) 5. 
I for along time thought that the Spirifera attenuata of Sowerby might be considered a distinct species 
by its smaller size, greater convexity of both valves, suddenly attenuated cardinal angles, strong pair of sulci 
defining the mesial ridge, more uniform, less branched and less fasciculated, and slightly smaller strix ; but 
experience shews me (as MM. de Koninck and Murchison, de Verneuil and Keyserling have already found) that 
these characters do not occur together constantly, but are individual peculiarities, really affording no character 
not found in the large S. striata, with the exception of the spinose cardinal angles occasionally found in the 
S. attenuata, but which from analogy we should not expect in old individuals. The large number of examples 
from Derbyshire of the true S. striata, which I have lately examined, shew that in those rare cases where the 
surface is preserved, the beautiful reticular sculpturing which I have figured in the S. clathrata may be found 
with a lens, and where it is absent the surface has manifestly lost some layers of shell; it is thus possible, that 
as this character has been hitherto overlooked in S. striata, that S. clathrata should be united thereto. Pune- 
