BIVALVIA. 45 
Genus—Isoarca, Munster. 
Shell equivalve, ventricose ; umbones large, anterior or antero-mesial, sometimes more 
or less spiral, ligament external ; hinge-border lengthened, curved, with two series of small 
transverse teeth, which decrease in size towards the centre ; pallial impression simple. 
Isoarca Scarsuregnsis, Lyc. Tab. XXXIX, figs. 5, 5a. 
Testa tenui, ovato-oblonga, tumida, umbonibus magnis subanticis, prominentibus, latere 
antico brevi, curvato, postico elongato, oblique declivi, sulcis duobus evanescentibus ; margine 
inferiore subrecto ; valvis striis concentricis irregularibus, inferne semel granulis irregularibus 
instructis. 
Shell thin, ovately oblong, somewhat inflated; umbones large, prominent, placed 
anterior to the middle of the valves, directed obliquely forwards ; anterior side short and 
curved elliptically, posterior side lengthened, the margin sloping obliquely downwards with 
two slightly impressed oblique furrows, which are distinct towards the umbo ; the lower 
border is lengthened and nearly straight ; the surface has fine irregular, concentric striations, 
and the sides have towards their lower border irregular, rounded granules, rather distantly 
arranged. 
Dimensions.—Length, 24 lines ; height, 16 lines ; diameter through the valves, 12 lines. 
A fine and rare example of a genus which seldom occurs in the Jurassic rocks of this 
country. It is much less inflated, and the umbones are more prominent than in Jsocardia 
transversa, Munst., less oblong, and with the anterior side more produced than in 
Tsoarca Lochensis, Quenst., and Isoarca eminens, Quenst. Other Jurassic species more 
remotely allied and approaching the orbicular figure are Jsoarca subspirata, Munst., 
Tsoarca texata, Munst., Isoarca decussata, Munst., and Isoarca cordiformis, Quenst. The 
general figure resembles Cypricardia, but when the hinge cannot be exposed, the genus 
may be distinguished by the granulations upon the surface, and by the tenuity of the test. 
Geological Position and Locality. The Cornbrash of Scarborough, in the collection of 
Mr. Leckenby. 
Leva Aneuica, D’Ord. Tab. XXXIX, fig. 7. 
NucvLa LAcHRYMA (obtuse variety) PAzl. Geol. York., i, pl. 9, fig. 25. 
Leva Anettca, D’Orb. Prodr., p. 275. 
Testa levigata parva, inflata, subtrigona, umbonibus medianis obtusis incurvis depressis, 
latere antico curvato, oblique-declivi, latere postico abrupte declivi, acute carinata, carina 
obliqua promimula ; valvis striis longitudinalibus obscuris et plicis inerementi paucis impressis. 
