122 MOLLUSCA PROM THE GREAT OOLITE. 



Pholadomya socialis. Tab. XI, fig. 7, la. 



Testa nucleo mediocre magnitudini, ovato ventricosd, latere antico brevi et gibboso, 

 latere postico elongate-, attenuato, et Jiianie ; umbonibus anticis magnis ; area cardinali 

 elongatd et planatd ; marginibus anticis et posticis curvatis j plicis longitudinalibus magnis 

 irregularibus ; costalis radiantibus (circa 6) obscuris, aut evanescenlibus. 



Shell with the nucleus moderately large, ovately ventricose ; anterior side short and 

 gibbose ; posterior side elongated, attenuated, and gaping, with a lengthened but narrow 

 aperture extending upon the superior and posterior border to its junction with the hinge 

 border ; upon the anterior border there is scarcely any perceptible aperture. The umbones, 

 which are placed anteriorly, are moderately large ; the cardinal area is lengthened and 

 rather flattened; both the anterior and posterior extremities are rounded, and pass 

 insensibly into the superior and inferior borders. The longitudinal plications are large 

 and irregular, with deep furrows between them, but they become less prominent, and are 

 almost lost as they approach the posterior extremity. The radiating little costse are 

 distinct only upon the umbones ; they are about six in number, but not unfrequently they 

 are absent altogether. 



This species presents its full share of variations of figure, not unfrequently the anterior 

 side appears compressed, and forms an obscure angle or rib, extending from the umbones 

 to the inferior border ; we have never seen the test preserved, but the nuclei display all the 

 more delicate features of the shell ; there are no traces of muscular or pallial impressions. 

 It was eminently gregareous, and occurred in a bed of buff-coloured calcareous sandstone, 

 situated nearly 100 feet above the fullers' earth, and associated with Lucina Orbigniana, 

 Ceromya Sgmondsii, Ceromya undulata, and other characteristic forms. 



The examination of a large number of specimens has enabled us to affirm its distinctness 

 from the P. laviuscula, Agassiz ; a shell which is not so much elongated and attenuated 

 posteriorly, and whose radiating costse, though delicate, are visible over the sides of the 

 shell, even to the inferior border. The large plications and more ventricose form distinguish 

 it from Pholadomya inornata, Sow., ' Geol. Trans.' 2d ser., vol. v, pi. xxi ; other species are 

 more distantly allied. 



Localities. — Small road side excavations two miles east of Minchinhampton ; Blisworth, 

 Northamptonshire. 



Pholadomya ovulum, Ag. Tab. XIII, fig. 12. 



Testa ovatd; umbonibus magnis anticis subdepressis, latere antico brevi, convexo, margine 

 rotundo, latere postico elongato et attenuato ; aperturd parvd, margine ligamenti oblique 

 declivi, inferiore curvato; lateribus costis radiantibus distantibus paucis, plicis longitudinalibus 

 impressis. 



