Bracuiopopa. } LOWER PALAZOZOIC MOLLUSCA. 247 
back, at one-fourth the length from the beak; and two thick, short, slightly-diverging cardinal teeth and 
lamellze: casts of entering valve shew two very short, thick, diverging pits of cardinal teeth; and a large 
double ovate pit, left by the great bifid rostral tooth, and a trace of a slender mesial septum. Average 
width one inch, proportional length of receiving valve =, of entering valve =, depth ©. 
This is so extremely like the Spirifer radiatus of the Wenlock limestone, that I have no doubt it has 
very often been confounded with it; and probably all the examples quoted by authors of S. radiatus, from 
the Caradoc and inferior strata, will be found referrible rather to the present shell, which abounds in these 
inferior rocks, where I have never seen the true S. radiatus (nor does this species occur to my knowledge in the 
Upper Silurian strata). Externally the present shell may be distinguished from the S. radiatus by its longer 
hinge-line, flatter sides, and coarser striation; while the internal cast shews the generic difference by demon- 
strating the pit for the great rostral tooth in the beak of entering valve, and the slender, tubular filling of 
the apical foramen, as in the Leptena (Strophomena) sulcata (Vern.), Bull. de la Soe. Géol. de France, 
2nd Series, Vol. V. p. 31. f. 4, to which it is in truth most nearly allied, but from which it differs by its greater 
gibbosity, and very prominent mesial ridge and hollow, extending from the beak, giving it exactly the appearance 
of a Spirifer. The Orthis vespertilio, when very finely sulcated, might be mistaken for this species, but has the 
mesial depression in the entering, and mesial elevation in the receiving valve, in which also the triangular 
foramen is open throughout. 
Position and Locality,— Extremely common in the impure Bala limestone of Moel y Garth, Welchpool, 
Montgomeryshire ; very abundant in the Bala limestone and schists of Bala, Merionethshire; very common 
in the Caradoc sandstone of Horderly; very abundant in the Bala schists of Alt yr Anker, Meifod, Mont- 
gomeryshire; Bala schists of Tan y Bwlch y Groes, S. of Bala, Merionethshire; Bala schists, Bryn Melyn, 
and Gelli Grin, Bala, Merionethshire; Bala schists S.W. of Pwllheli, Caernarvonshire; Bala schists of Pen y 
Gaer, Cerrig y Druidion, N. Wales; Bala schists of Beaver’s Grove, Bettws-y-Coed, N. Wales; Bala schists 
of Rhiwargor, near Llanwddyn, Montgomeryshire ; Bala schists of Gaer Fawr, Welchpool, Montgomeryshire ; 
Bala schists of Bryn Hithin, Penmachno, N. Wales; Bala schists, Garnedd Uchaf, W.of Bala Lake; Bala 
limestone, Dinas Mowddy, Merionethshire; Bala limestone of Coniston, Lancashire; Bala schists of Bwlch y Groes. 
Sub-genus. LEPTAGONIA (AM‘Coy)*. 
Gen. Char.—Shell rhomboidal ; rostral portion of both valves flattened and concentrically wrinkled ; the 
margin of both valves abruptly bent downwards at right angles to the flattened rostral portion; texture minutely 
but not closely punctured; surface longitudinally striated, generally without spines; cardinal area distinct, 
narrow, rhomboidal, formed of both valves; a wide deltoidal opening in the convex or receiving valve, nearly 
filled by the projection of a rhomboidal two- or four-lobed boss on the beak of the entering valve, leaving 
a small perforation for a pedicle, separated by a small pseudo-deltidium from the beak : internally, strongly 
punctured in the rostral portion, the front with dichotomising impressions of the pallial vessel: the receiving 
valve has its opening bordered by two strong diverging cardinal teeth, from which a semicircular ridge curves on 
each side, into a small longitudinal mesial septum, bounding the rounded, or triangular muscular impressions ; 
the included spaces often strongly marked with radiating sulci; entering valve with a small depression below the 
projecting beak, for insertion of the adductors ; from this the mesial septum extends, leaving a circular impres- 
sion with raised margin of the large anterior adductors close on each side; anterior to which, at the end of 
the septum, near the middle of the shell, are two close, longitudinal pear-shaped impressions of the anterior 
adductors, also with raised margins. 
* See remarks on Leptena, page 232. 
