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while in other specimens, and especially in very large individuals, the principal ribs 
get gradually stronger as they extend from the beaks to about the middle of the length 
of the valve, where they again become gradually smaller or m6re attenuated as they 
extend to the margin, and where much less difference or more uniformity in ^ize prevails 
between the principal and intermediate riblets ; and, indeed, near the margin, in many 
specimens the principal riblets and the interpolated ones are of about equal strength and 
closeness. 
I have already described and illustrated, in PI, XXVI, figs. 2 e, 4 a, the manner in 
which both ribs and interspaces are crossed by equidistant concentric ridges or scale-like, 
raised lines of growth. In some specimens, however, the riblets seem smooth, and the 
interspaces alone crossed by concentric rounded ridges. 
Malformations in this species, as well as in its named varieties, are often found, and 
of these I give two illustrations (Sup., PI. XXXVII, figs. 4, 5) from specimens obtained 
by Mr. C. W. Peach at Chapel Hill, Kirkald, and at Abbey Craig Quarry, west of the 
Bridge of Allan, in Scotland. 
29. Streptorhynchus crenistria, var. senilis, Fhillips. Dav., Carb. Mon., PI. 
XXVII, figs. 2—10. 
Some palaeontologists seem disposed to erect this important variety into a distinct 
species, and it cannot be denied that in some localities, particularly in Australia, it 
alone occurs. The convexity of its valves, usually large area, and shorter hinge-line 
distinguish it from the typical St. crenistria. 
30. Streptorhynchus crenistria, var. cylindrica, J/'Coj/. Dav., Carb. Mon., p. 128, 
PI. XXVII, fig. 9 ; and Sup., 
PI. XXXVII, figs. 6, 7. 
Stkeptorhynchus ckenistria, var. eobusta, Hall. James Thomson, Traus. Geol. 
Soc. of Glasgow, vol. ii, 1^63. 
— — — James Neilson, jun., Trans. 
Geol. Soc. of Glasgow, 1875. 
At p. 128 of Monograph I described this variety, and gave a figure of Prof. 
M'Coy's typical example. On attentively comparing the specimens described and figured 
by Mr. James Thomson/ from the neighbourhood of Campbeltown, under the name of 
' "On the Geology on the Campbeltown District," 'Trans, of the Geological Society of Glasgow/ 
vol. ii, pi. ii, fig. 1, 1863. 
