196 



CONCHIFERA. 



IsOCARDIA. 



of the foot of a horse, which suggested the idea of its generic 

 name. 



Found in the Upper Lias, Cheltenham, at Toddenbam, near 

 Shipson-on-Stour, and near Oxford. 



Genus XVI.— MEGALODON.— 5owerJy. 



Shell bivalve, equivalve, longitudinal, acuminated towards 

 the beaks ; a large bifid tooth placed upon a septum across 

 the beak of the right valve, and one irregular and more acute 

 tooth, similarly situate in the left valve ; a small pit near the 

 teeth for the reception of the ligament, which is anterior, long, 

 and external. 



1. Megalodon cucullatus. — The Hooded Megalodon, 

 pi. 79, figs. 13, 14. 



M. CHCul/atiis. Sowerby, VI. p. 132, pi. 56'8. 



Oblong, smooth, convex, thick, ponderous ; beaks pointed, 

 incurved ; a deep, acute-edged pit, for the reception of the 

 anterior muscle, situate close to the thick plate on which 

 the hinge-teeth are situated. 



Limestone, Bradley, near Newton Bushel, Devonshire. 



2. Megalodon carinatus. — The Keeled Megalodon, pi. 

 LXXXIII. figs. 21, 22. 



M. carinatus. Goldfuss, pi. 132, fig. 9 ; Phillips, Pal. 

 Foss. p. 136, pi. 60, fig. 60*. 



Transversely elongated, oblique ; sub-lobate anteriorly ; 

 surface diagonally carinated and elevated ; beaks recurved 

 over the small and rather deep lunule ; surface with oblique 

 and prominent lines of growth. 



In the Devonian Rocks, Newton, and Ogwall. 



Genus XVII.— ISOCARDIA.—Lamarci-. 



Shell equivalve, heart-shaped, ventricose ; beaks very dis- 

 tant, divergent, and involute ; hinge with two primary com- 

 pressed teeth in each valve, the one next the apex inflected 

 under the umbo ; and witii one elongated, lateral tooth, situ- 

 ate immediately before the ligament, wliich is external, and 

 divided into two segments at its posterior extremity, both of 

 which are divergent to the point of the beak in each valve ; 

 both valves provided with two lateral, remote, muscular im- 

 pressions, the linear impression of the mantle is entire, and 

 extending from one muscular impression to the other. 



1. Isocardia minima. — The Very Small Isocardia, pi. 

 LXXX. figs. 2, 3. 



/. minima. Sowerby, III. p. 171, pi. 295, fig. 1. Phillips, 

 Geo. York, I. pi. U, fig. 40. 



Sub-deltoidal, globose ; anterior side a little truncated ; 

 posterior side somewhat flattened and heart-shaped. 



The Cornbrash, Scarborough and Wiltshire. 



2. Isocardia abrupta. — The Abrupt Isocardia, pi. 

 LXXX. fig. 7. 



Cardita abrupta. Sowerby, I. p. 200, pi. 89, fig 2. 



Triangular, inflated ; anterior side nearly straight ; beaks 

 produced near one of the angles ; surface covered with oblique 

 strias, some of which meet the longitudinal ones upon the an- 

 terior side ; the others terminate abruptly on the first trans- 



verse rib ; the anterior side longitudinally striated ; front 

 provided with five or six longitudinal ridges. 

 Inferior Oolite, Swanwick, Somersetshire. 



3. Isocardia striata. — The Striated Isocardia, pi. 

 LXXX. fig. 4. 



Cardita striata. Sowerby, I. p. 199, pi. 89, fig. 1. 



Nearly rectangular and quadrangular, much inflated ; beaks 

 situated near to one of the angles ; anterior side rounded ; the 

 whole surface with longitudinal curvilinear striae. 



Inferior Oolite, Swanwick, Somersetshire. 



4. Isocardia tener. — The Tender Isocardia, pi. LXXIX. 

 figs. 26, 27. 



/. tener. Sowerby, HI. p. 171, pi. 295, fig. 2. 



Obovate, anteriorly sub-truncated, and {posteriorly rounded ; 

 beaks produced ; surface smooth ; the anterior side is circum- 

 scribed by a slight ridge, which has bent striae projecting some- 

 what beyond it ; texture of the shell thin. 



The Kelloways Rock, Kelloway and Wiltshire. 



5. Isocardia rostrata. — The Beaked Isocardia, pi. 

 LXXIX. figs. 18, 19. 



/. sulcata. Sowerby, III. p. 172, pi. 295, fig. 3. 



Very ventricose, deltoidal ; anterior side produced and 

 acute ; posterior side depressed and rounded ; beaks short ; 

 size of a hazel nut. 



The Inferior Oolite, Cotswold and Yorkshire. 



6. Isocardia sulcata. — The Furrowed Isocardia, pi. 

 LXXIX. figs. 22, 23. 



/. sulcata. Sowerby, III. p. 172, pi. 295, fig. 4. 



Orbicular, its depth exceeding its width ; beaks remote, 

 much incurved, with a heart-shaped depression beneath them ; 

 surface pearlaceous, with longitudinal, broad, numerous fur- 

 rows. 



The London Clay, Islington and Sheppy, 



7. Isocardia concentrica. — The Concentric Isocardia, pi. 

 LXXIX. figs. 28, 29. 



/. concentrica. Sowerby, \. p. 147, pi. 491, fig. 1. 



Oval, heart-shaped, transversely elongated, the depth and 

 length nearly equal, and considerably less than the width ; 

 beaks projecting and incurved ; surface smooth, with numerous 

 shallow, transverse furrows, which become more distant as 

 they approach the edge ; shell thin. 



The Fuller's Earth, Widcombe, and Great Oolite, Bath. 



8. Isocardia similis. — The Similar Isocardia, pi. LXXX. 

 fig. 12. 



I. similis. Sowerby, VI. p. 27, pi. 516, fig. 1. 



Ventricose, transversely oblong ; middle of the disk slightly 

 flattened ; anterior side small, turned a little up ; base nearly 

 straight ; surface with very shallow transverse undulations. 



The Lower Greensand, Sandgate, near Margate. 



9. Isocardia Cor. — The Heart Isocardia, pi. LXXX. f 5. 

 /. cor. (?) Sowerby, VI. p. 27, pi. 516, fig. 2. Brown, 



Illust. Recent Conch. Brit. &c. p. 86, pi. 30, fig. 9, and pi. 

 30*, fig. 5. Chama cor. Montagu, p. 134. 



Heart-shaped, greatly inflated, with strong, irregular, trans- 

 verse wrinkles, which increase in coarseness and elevation as 

 they approach the base ; beaks much curved and acute. 



The Red Crag, Sutton, and the Coral Crag, Ramshot. 



10. Isocardia triangularis. — The Triangular Isocardia, 

 pi. LXXX. fig. 11. 



