364 G. B. Pritchard : New Slhiriaii Hiralrc. 



the front of its centre line uniljo-venti-ally, hut liveonics notably 

 flattened towards the anterior margin as well as towards the pos- 

 terior keel. Posterior keel only faintly defined iiiuljunally, but 

 rapidly increasing in strength posteriorly, till it foinis an excep- 

 tionally strong ridge margining the depressed posterioi- area. Shell 

 surface finely concentrically sculptured, running about three ridges 

 in two millimetres in the middle region, apparently raised into 

 slight frills on the posterior keel; the frills are strongest post-ven- 

 trally, l)ut tlie preservation of the specimen is not so perfect as to 

 fully show the original extent of this feature. No radial marking 

 is aj)i:)arent. Interior of valve deeply concave umbonally and 

 running out shallower towards the ventral margin, which is bevelled 

 off to a general acute edge, with a suspicion of faint denticulation. 

 The hinge has been cleared of matrix, but the cardinal teeth are 

 evidently obsolete; there is a faint suspicion of an anterior lateral 

 tooth, whilst to the posterior a broad shallow area for the reception 

 of the internal ligament is margined by an elongated nariow ridge. 

 Anterior adductor muscular scar very large, f)vate. and showing a 

 distinct and elevated callous rim towards the ventral margin, callous 

 failing out dorsally. Posterior adductor muscular scar much 

 smaller, narrowly elongate and margined anteriorly with a strong 

 callous ridge. Pallial line entire and narrow. Internally there is 

 some evidence of a radial structure wliiih jxobably gave rise to a 

 slight denticulation of the ventral margin, but this is not very 

 distinctly preserved in the present specimen. 



Dimensio7}s. — Antero-posterior diameter, G8 mm. ; und)o-venti-al 

 diameter, fi2 mm.; greatest thickness through one valve. 15 mm.; 

 thickness of shell about 2 mm. 



Locdlifl/. — Cave Hill Quarries, Lilydale. Siliiii.in limestiuie 

 fauna. Collected by S. H. Mitchell. 



1 desire to express my best thanks to Mr. L. Knibhs for the 

 ])hotogra})hs which illustrate this shell. 



EXPLAN.\T10X OF PL.VTK .\X1X. 



Fig. 1. E.xteriial view of valve, about two-thirds natural size. 

 Fig. 2. internal vIcav of valve, about two-thirds natural si/.e. 

 Fig. 3. Unibonal aspect, a little less than natuial size. 



