BIVALVIA. 45 



Genus — Isoarca, Munstcr. 



Shell equivalvc, ventricose ; umboncs large, anterior or antero-inesial, 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 Scarburgensis, Lye. Tab. XXXIX, figs. 5, 5 a. 



Testa tenui, ovato-ohlonga, tumida, umbonibits magnis subanticis, prominentibus, latere 

 antico brevi, curvato, postico elongaio, oblique declivi, sulcis duobus evanescentibus ; margine 

 inferiore subrecto ; valvis striis concentricis irregularibus, in/erne 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 Isocardia 

 transversa, Munst., less oblong, and with the anterior side more produced than in 

 Isoarca Lochensis, Quenst., and Isoarca eminens, Quenst. Other Jurassic species more 

 remotely allied and approaching the orbicular figure are Isoarca subspirata, Munst., 

 Isoarca 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. 



Leda Anglica, B'Orb. Tab. XXXIX, fig. 7. 



NucuLA LACHRYMA (obtuse Variety) Phil. Geol. York., i, pi. 9, fig. 2.5. 

 Leda Anglica, D'Orb. Prodr., p. 275. 



Testa Icevigata parva, injlaia, 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 iricrementi paucis impressis. 



