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Dimensions . — Length, twenty-three millimetres ; width, thirty-four 
millimetres ; thickness, eigliteen millimetres. 
Horizon and Localities . — I have not been able to find any tangible 
difference between the Spanish specimens of this species from the limestone 
of Perones and that wdiicli has been collected by Mr. Clarke in a black lime- 
stone from the Yass District. It is probable that both belong to the same 
geological epoch, that is the Middle Devonian, 
4. Spirifer yassensis, W. B. Clarke. 
PI. Ill, Pig. G. 
Shell snbglobose, almost as broad as long. Ventral valve regularly 
curved on itself ; twice as deep as the opposite valve ; beak fairly strongly 
curved on itself and pointed ; area rather higher, hollowed like a gutter, ends 
slightly rounded ; sinus deep, very wide in proportion to the size of the shell, 
and angular at the base. Dorsal valve provided with a prominent fold 
corresponding to the sinus of the oj)posite valve ; surface of the two valves 
ornamented on either side by fourteen or fifteen radiating ribs, of which the 
last are very small and sometimes strongly separated by furrows of the same 
width as the adjoining ribs. On good specimens several lines of growth may 
be seen towards the borders. 
Dimensions. — A very well-preserved specimen gave the following 
measurements: — Length, nineteen millimetres; width, twenty millimetres; 
thickness, fifteen millimetres ; width of the sinus, seven millimetres. 
Belations and Differences. — It is only after great hesitation, minute 
examination, and numerous comparisons that I have decided to separate this 
species from Spirifer hystericus, Schlotheim, subspeciosus and Glincanus, 
de Verneuil, to which it has the greatest resemblance. It differs from the 
first two in its less broad shape and in the number and less thickness of the 
ribs as well as in the stronger curvature of the beak. One can still less 
confuse it with the last mentioned, whose greatest width is towards the 
middle, and in which the very flattened folds are twice as wide as the furrows 
that separate them. 
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