492 BRITISH PALASOZOIC FOSSILS. [ LAMELLIBRANCHIATA. 
line; no hinge-teeth, but a triangular septum in the cavity of each beak, parallel with the plane of the lateral 
margins (leaving deep slits under the beaks of the casts); dimyarian, each adductor surmounted by a small 
accessory impression, the anterior adductor attached to the inner face of the rostral septum. 
The width of the cartilage facet, and the great inequality of the valves, separate this paleeozoic genus from 
even those of Myili, which have a similar internal septum in the beak; but from its affinity in other respects 
it helps to blend the two groups of Pleuroconcha and Jsedrolotila. The small impression over the large 
anterior adductor I find (in the recent Mytili with rostral plates) ts the scar of the insertion of the adductor 
from the opposite valve, instead of retractors of the foot, as commonly supposed, the larger impression being 
the scar of the origin thereof. 
MYALINA ACUMINATA (Som. Sp.) 
Ref. and Syn. = Modiola acuminata Sow. Geol. Trans. 2nd Series, Vol. III. p. 119. = Mytilus septifer King, 
Perm. Foss. t. 14. f. 8 to 13 (= Ostrea? pusilla + O? Tayloriana id. olim). 
Desc.—Deltoidal; beaks tumid, narrow, moderately curved; greatest depth a little in front of the middle, 
from whence the surface slopes, very abruptly, or almost vertically to the ventral margin, and gradually to the 
posterior end and broadly dilated wing ; hinge-line of moderate length, elevated, the anal angle obtuse ; poste- 
rior end very wide, convex ; anterior end projecting slightly in front of the beaks as in Modiola, and with a very 
small byssal sinus a little in front of the middle of the ventral margin; surface marked with moderately strong, 
irregularly-unequal, slightly imbricating laminze of growth ; rostral plate in the casts moderately strong, bearing 
the comma-shaped impressions for the origin and insertion of the adductor muscles ; two or three minute retractor 
impressions at the apex of the beak. Length from beak to respiratory posterior margin nine lines, proportional 
length of hinge-line from beak to anal angle =, greatest width from anal angle to ventral margin “, depth of 
left valve =. 
According to Prof. King, this species increases in proportional width by age, but it is at all times easily 
distinguished from the J/. sywamosa by the very much wider and more obtuse form; the larger anterior end, 
more eleyated and compressed hinge-margin, and the less rugged, concentric imbrications of the surface. 
Position and Locality—Not uncommon in the Permian magnesian limestone of Humbleton. 
Myatrna squamosa (Sov. Sp.) 
Ref. and Syn. = Mytilus squamosus Sow. Geol. Trans. 2nd Series, Vol. III. p. 120. = Mytilus Hausmanni 
Gold. Pet. t. 138. f. 4. = Mytilus squamosus King, Perm. Foss. t. 14. f. 1 to 7. 
Desc.—Obliquely ovato-trigonal ; anterior end tapering to the very large, prominent, pointed, widely sepa- 
rated beaks; valves very gibbous from the beaks towards the posterior end, which is semielliptically rounded ; 
ventral margin very obtuse, from the abrupt arching over of the valves nearly at right angles to the plane of the 
margins ; hinge-line slightly elevated; cardinal angle very obtuse, wing moderately and gradually compressed ; 
byssal sinus in the ventral margin very small, slightly in front of the middle of the length; surface marked with 
very coarse, irregular, concentric, prominent, imbricating strize and interruptions of growth. Average length 
from beak to posterior end nine lines, in proportion thereto length of hinge-line ==, greatest width from cardinal 
angle to opposite ventral margin =, depth of left valve (greatest a little in front of the middle) ;%, depth of 
right valve =. 
This species is easily distinguished by the extreme coarseness and irregularity of its concentric strice 
and imbrications of growth. When imperfectly preserved the surface appears more even and smoother, and 
then approaches very closely to some of the carboniferous species. 
Position and Locality —Common in the Permian limestone of Humbleton, 
