LAMELLIBRANCHIATA. | UPPER PALZOZOIC MOLLUSCA, 5]1 
Macropon striatus (Schlot. Sp.) 
Ref. and Syn.= Mytilus striatus Schlot. Miinch. Akad.Vol. VI. t. 6. f. 3. = Cuculleea sulcata Sow. Geol. Trans. 
2nd Series, Vol. III. p. 119. (according to King) = Arca antiqua (Miinst.) Gold. Petref. t. 122. f. 8. 
=Arca Loftusiana Howse, Trans. T. N. F. C. Vol. I. p. 246.= Arca tumida Sow. M. C. t. 474. f. 3. 
= Byssoarca striata (Schlot. sp.) + B. tumida (Sow. sp.) King, Perm. Foss. t. 15. f. 1 to 9? + Arca Kingiana 
(de Vern.) King, Perm. Foss. t. 15. f. 10, 11, 12. 
Desc.—Shell oblong, subrhomboidal, very gibbous when old, less so when young: beaks very prominent, 
incurved ; separated by a triangular area in each valve, increasing in width with age, and marked by the 
fine cartilage sulci, angulated beneath the beak ; anterior side short, nearly rectangular, compressed at the angle, 
moderately rounded; ventral margin only slightly convex, with a variable byssal sinus in its anterior third; 
posterior end obliquely truncated, very slightly sigmoid; posterior slope rather flattened, very abruptly com- 
pressed, a strong sulcus separating it from the prominent, obtuse, diagonal ridge extending from the beak 
to the respiratory angle; entire surface covered with coarse, narrow, filiform ridges, usually simple, but occa- 
sionally bifurcate, crossed by strong concentric scaly lines of growth, rendering them transversely nodular under 
the lens when well preserved ; there are also several irregular stronger interruptions of growth, coarser and more 
numerous near the margins of old specimens ; the intervening sulci are deep, and almost as wide as the ridges. 
571 
Average length ten lines, proportional width from beak to ventral margin *%, length of anterior end from 
53 85 
beak =, length of hinge-line from beak =, from beak to respiratory angle &, width of posterior end =, depth 
of both valves =; five ridges in the space of two lines on the middle of the shell at two lines from the beak. 
I have very carefully examined a considerable suite of specimens of this fossil, including specimens named 
by Prof. King Byssoarca tumida and B. striata, and I can state positively that the differences between those 
species are merely the natural changes produced by age in the genus, that is to say, in any series of specimens 
the larger are always much more tumid, more round in the ventral margin, and rather shorter proportionally than 
the smaller; the ridging being perfectly the same in both, and the width of the hinge-area separating the beaks 
increasing regularly with the size, age, and tumidity of the specimens. It is only because I have not seen the 
specimens themselves, that I hesitate to quote the 4rca Kingiana (de Verneuil) as the young of this species, as 
Prof. King has found in it the sulcation of the present fossil, and distinguishes it only by its smaller size, 
narrower cardinal area, and other characters which collectively agree in the indication of youth. 
Position and Locality.—V ery common in the Permian magnesian limestone of Humbleton. 
Family. NUCULID. See page 285. 
Genus. NUCULA. See page 285. 
NucuLa? ATTENUATA (F/lem.) 
Ref. and Syn.—Fleming, Brit. Anim. p. 403.=Nucula claviformis (Sow.) Phil. Geol. York. Vol. II. t. 5. 
f. 17, (not of Sow.). 
Desc.—Clavate, anterior side of moderate length, regularly rounded semielliptically from the beak to 
opposite ventral margin ; posterior end elongate, gradually compressed, tapering to an obtuse point; beaks very 
small, much incurved, not prominent ; anterior lunette elongate, elliptical, small ; posterior lunette or corslet very 
narrow, steeply sloped, strongly marked, extending nearly to the end of the shell, bounded by a prominent 
obtusely rounded ridge from the beak to the posterior extremity; valves gently and evenly convex, most so at 
equal distance from the beak, anterior end, and ventral margin. Surface sculptured with extremely regular, fine, 
concentric, thread-like, elevated ridges (ten or eleven in one line at three lines from the beak), extending from 
the anterior margin, as far as the obtuse ridge bounding the posterior lunette, which ridge is left nearly- 
smooth by their abrupt cessation. Average length about ten lines, proportional width from beak to opposite 
30 
ventral margin ;;, length of anterior end 4, depth of one valve <4. 
[rasc, 111. ] 3U 
