80 
Spirifer Murcllisonicmiis, L. G-. de Kouiuck, 18d3, Precis Elem. Geol. par d’Omalius 
d’HalloT, p. 523. 
jj ,, De Verneuil, 1845, Eussia and the Ural Mts., II, p. IGO, pi. 4, 
J.. 
,, disjlinctus, Idem, 1845, ibid., p. 157, pi. 4, fig. 4. 
,, Archiaoi, Idem, 1845, ibid., p. 155, pi. 4, fig. G. 
,, l^cirumeusis, Sowerhy, M S., 18G3, Salter, Quart. Journ. Geol. Soc., XIX, p. 480. 
Splldfera disjwicta, T. Davidson, 18G5, Mon. Brit. Dev. Brach., p. 23, pi. 5, figs. 1-12, and 
pi. G, figs. 1-5. 
,, ,, J. Hall, 1SG7, Pal. X. York, IV, p. 243, pi. 41, 42, and pi. G3, fig. 14. 
The shell of this species is very subject to variation ; although it 
usually is broader than long, it sometimes happens that the contrary obtains, 
and then it assumes the shape to tvliich thirty years ago I gave the name 
S. IIurcMsonianus. In all cases the valves are convex, sometimes even 
gibbous and almost of equal depth. The beak of the ventral valve is 
usually prominent, little incurved ; area more or less elevated, concave, and 
hollowed; fissure triangular, partially covered by a pseudo-deltidium made up 
of two plates ; sinus rather deep and clearly marked off. The mesial fold of 
the dorsal valve is raised in proportion to the depth of the sinus of the 
other valve and separated from the rest of the shell by two furrows a little 
better marked than those that separate the other surface ribs. Each of the 
valves is ornamented on either side by twenty to forty ribs, simple, rounded, 
and radiating, the thickness of which decreases in proportion to their distance 
from the sinus or the mesial fold ; the ribs in the two last-mentioned regions 
are thinner than those of the lateral portions ; their number increased by 
bifurcation, or by the insertion of anew fold, at variable distances from the 
beaks. In well preserved individuals the ribs are crossed by small concentric 
growth strire. 
The shell of the ventral valve is usually thicker than that of the 
dorsal ; the muscular impressions in it are rather deeply excavated, while 
those of the other valve are almost superficial. 
Jlimensions. — Very variable; some varieties may attain a width of 
about ten centimetres, by a length of three to four centimetres. The average 
■width of Australian specimens is three and a half centimetres by a length of 
two centimetres, 
