LAMELLIBRANCHIATA. | LOWER PALAXOZOIC MOLLUSCA. 267 
depth of one valve (about one-third the length from the beak) %. Surface with minute, irregular plications 
and lines of growth. 
Distinguished from all the varieties of the MZ. modiolaris by its greater gibbosity, shorter hinge-line, and 
broader anterior end. 
Position and Locality—Bala schists of Pen Cerrig, Serth, (very common); Bala schists of Cymmerig, 
E. of Bala, Merionethshire. 
Explanation of Figure.—P\. 1. 1. fig. 16. Left valve, natural size, from Pen Cerrig. 
MoproLorsis MODIOLARIS (Conrad Sp.) Pl. 1.1. fig. 17 and 18. 
Ref. and Syn. = Pterinea and Cypricardites Modiolaris + C. angustifrons + C. ovatus Conrad ; = Modiolopsis 
Modiolaris Hall, Pal. N. York, t. 82. 
Sp. Ch.—Obliquely ovate, gradually widening to the posterior end, about two and half to three times longer 
than wide; a diagonal, slightly sigmoid gibbosity, from the small prominent beaks becoming gradually obsolete 
towards the inferior posterior angle; anterior end one-eighth to one-ninth of the length, narrow, rounded, 
often shewing a round boss indicating the insertion of the large anterior adductor; a shallow sinus in the 
anterior part of the ventral margin, from which a slight shallow hollow extends towards the beak; posterior 
end wide, elliptical, obliquely subtruncate, posterior slope gradually compressed ; hinge-margin elevated, straight, 
or slightly convex, with a slender, internal cartilage plate a little below it, and extending about one-fourth of the 
length from the beaks to the posterior end; surface with obsolete concentric lines and waves of growth. 
Length two inches one line; width from beak to opposite margin 4; greatest width (at end of hinge-line) 5 ; 
greatest depth (at one-third from the beak along the diagonal ridge) varying from half to one-third of the width. 
This differs from the M. expansa (Port.) in its smaller anterior lobe, more obtuse diagonal gibbosity, 
sinuate ventral margin, and shorter and more elevated hinge-line. 
Position and Locality—Not uncommon in the Caradoc sandstone of Horderly; also in Bala schist at 
Maes Hir, N. E. of Aber Hirnant, N. Wales; Bala schists of Cader Dinmael, near Corwen, Denbighshire. 
Explanation of Figures —P\. 1.1. fig. 17. Right valve of normal variety, natural size, from Cader Din- 
mael ; fig. 18, depressed variety from Horderly. 
MoproLopsis EXPANSA (Port. Sp. ?) 
Ref. = Modiola id. id. Geol. Rep. t. 33. f. 6. 
An imperfect specimen of the posterior end probably of this species has occurred in the blackish Bala 
schists of Cerrig y Druidion. It differs from the J7. Modiolaris in the dorsal and ventral margin being 
more nearly parallel, the posterior slope and sides flatter,‘and meeting at a more angular ridge extending to 
the posterior inferior angle ; the substance of the shell is also much thicker and the concentric lineation sharper 
and stronger. 
Moptotorsis Nritsoni (fis. Sp.) Pl. 1. I. fig. 21. 
Ref. and Syn.= Modiola Nilsoni Hisinger, Leth. Suec. t. 18. f. 13 = Modiola antiqua Sow. (not Goldf.) 
Sil. Syst. t. 18. f. 1 = Mytilus gradatus Salt. Mem. Geol. Sury. Vol. II. t. 20. f. 4. 
Sp. Ch.—Oblong, widening towards the anal angle, where the width is usually three-fifths of the length, 
rarely a little more, often something less; beaks small, narrow, prominent, anterior end about one-sixth of the 
length, elliptically rounded ; anterior adductor strongly marked at the posterior edge; valves compressed ; a 
narrow diagonal gibbosity of small amount extending from the beak towards the broad, obtusely-rounded, 
respiratory angle, from which the posterior slope gradually inclines to the hinge-margin, which is slightly elevated, 
and extends to about one-third the length of the shell from the posterior end (anal angle 120°) ; ventral margin 
nearly straight, with a wide, scarcely perceptible, sinus about the middle, from which a very shallow concavity 
MM 2 
