Bracuiovopa. | DEVONIAN MOLLUSCA. 379 
twice from thence to the margin without altering materially in thickness; four ridges in two lines in the 
middle of the shell, at an inch from the beak; no distinct regular scaly laminee of growth; muscular im- 
pressions in receiving valve forming an oblong defined space, rather longer than wide, extending about one- 
third the length of the shell. Length of average specimen one inch nine lines, proportional length of 
entering valve #4, proportional width 4, proportional depth of both valves =. 
By cautiously breaking a specimen in the right direction, I have had the pleasure of ascertaining the 
existence of the spiral appendages in this beautiful fossil, and to demonstrate their spiral convolution in the 
plane of the lateral margins, instead of at right angles thereto, as in the different groups of Spirifera; they 
are of rather small size, the diameter of the spires not exceeding half the width of the shell, so that they are 
not placed one behind the other, but each occupies its own side of the median line. The pallial vessels are 
large, composed of three or four main trunks on each side, branching once or twice to the margin. 
Position and Locality —Extremely abundant in the Devonian limestone of Plymouth and Newton Bushel; 
Devonian shale of Pridmouth, Menabilly; an obscure internal cast, probably of this species, from the schists 
of Fowey, Cornwall. 
SPIRIGERINA RETICULARIS (Linn. Sp.) See page 198. 
I cannot perceive the slightest difference between some of the Eifel and upper Silurian, and some of 
our Devonshire specimens of the first variety, specimens of which are in the collection from the reddish 
calcareous shales of Newton Bushel. 
Var. 3. ASPERA (Schlot. Sp.) 
Ref. and Syn. = Terebratula aspera Schlot. Min. Taschenbuch, Vol. VII. t. 1. f. 7. = Atrypa squamosa Sow. 
Geol. Trans. 2nd Series, Vol. V. t. 57. f. 1 and la + Spirifera affinis Sow. same plate, f. 11. 
This variety, which is at first sight so extremely different from the ordinary Silurian type, by its few very 
thick rounded ribs, with comparatively few branches, and distant, strong, concentric scales of growth, passes by 
the most insensible gradations into that species, when a large series either from the Eifel or Devon is examined. 
The most marked types of this peculiar form are nearly orbicular; the length and breadth equal; length about 
one inch, the depth half the length; the entering valve moderately convex along the middle, raised into a 
small rounded sinus at the margin; receiving valve slightly convex at the sides and near the beak, becoming 
depressed in the middle towards the margin: about sixteen thick, rounded, radiating ribs, two or three only 
of which dichotomise (two occupying a space of two lines at six lines from the beak, and the concentrie 
laminz nearly a line apart at the same distance). 
This highly typical form of the variety aspera is common in the Devonian limestone of Plymouth, and 
not uncommon in the Eifel; but with these in both localities occur specimens in which the depth in pro- 
portion to the length is =; the ridges become gradually smaller, more numerous, and more often branched, 
and the concentric laminze of growth become more crowded ; so that no character can be assigned, so far as 
my observations go, which would serve to separate specifically the intermediate varieties ; nevertheless I have 
not seen the var. aspera in Silurian rocks. The small triangular, flat, cardinal area may often be seen in 
the more depressed specimens, with small foramen separated by a deltidium from the hinge-line ; these how- 
ever being, as in the Hifel specimens, often concealed by the adpression of the beak. On examining the 
original specimen figured by Mr Sowerby as above, under the name Spirifera affinis, I find it is a young in- 
dividual of the present species var. aspera, shewing the cardinal area rather less than in his figure, and not 
greater than is often seen in Hifel specimens. 
Position and Locality of Var. aspera—Common in the Devonian limestone of Plymouth; Devonian 
shale of Pridmouth, Menabilly, and Fowey, Cornwall. 
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