78 SYNOPSIS OF THE CHARACTERS OF THE 



of thin liom and membrane, bending readily, and without injury in any direction, and capable of receiving 

 very slight impressions, but with sufficient elasticity to regain its original shape on the pressure being removed. 

 This seems to have been also the state of the PosidonicB, as they impress each other in all directions in the 

 shales without injury. The fine shales in which they occur must have been deposited in deep water, which is 

 also the habitation of the recent Jpli/sia. The large, proportionate size of the soft portion of the animal, when 

 compared with the small shell, would account for the general dark colour of the Posidonia shales. If the above 

 view were correct, the Posidonia should bo removed to the Gasteropoda. 



Posidonia Becheri. Gold. 



Posidonia Becheri. Gold. Pet. — Posidonia Becheri. Sow. Geol. Trans — Posidonia Becheri. Phil. Pal. Foss. 



Sp. CA.— Obovate, oblique, compressed; numerous, angular, sharp, concentric ridges ; posterior ear twice 

 the width of the anterior. 



This species varies considerably in its characters ; the number of the concentric ribs, and the outline, being 

 inconstant ; some specimens exhibit delicate striae parallel with the concentric ribs. Length two inches four 

 lines, width two inches three lines. 



Posidonia complanata. Portk. 



Posidonia complanata. Portk. Geol. Eep. 



Sp. C/i.- — Transversely elliptical, compressed ; beaks small, close to the anterior end, which is small, 

 rounded ; posterior end broader, rounded, ventral margin very convex ; surface striated concentrically. Length 

 from the beak to the posterior extremity one line. 



Thislittleshellbelongs very probably to the genus Zsawra, and, if so, it should be removed to the Crustacea. 



Posidonia cosTATA. M'Coy. (PI. XIII. fig. 15). 



Sp. Ch. — Ovate, oblique, compressed, four strong, rounded, radiating ribs occupy the middle of this shell; 

 no concentric wrinkles. 



This beautiful shell is the only Posidonia I know of without concentric ^vTinkles, it is rendered very striking 

 by its four longitudinal, distant, well-defined mesial ridges. Most of the specimens shew a minute oblique pli- 

 cation of the surface, but as this is not constant in its direction, it is probably due to the folding of the epidermis, 

 if it had one, or may be due to mechanical causes. Length ten lines, width eight lines. 



Posidonia lateralis. Sow. 



Posidonia lateralis. Sow. Geol. Trans Posidonia lateralis. Phil. Pal. Fos. 



Sp. C/;.— Transversely ovate, twice and a half as wide as long; beaks close to the anterior end; surface 

 with numerous, large, concentric wrinkles. 



Portions of this species have occurred in company with the P. Becheri, and P. tuberculata; its great 

 transverse diameter is the principal character. 



Posidonia MEMBRANACEA. M'Coy. (PL XIII. fig- 14). 



Sp. C/(.— Compressed, membranaceous, transversely elongate, recurved; dorsal and ventral margins nearly 

 parallel ; width three times the length ; surface with numerous minute, concentric, and longitudinal membra- 

 naceous wrinkles ; hinge-line short, beaks in the centre of it. 



This little species is exceedingly distinct from all others of the genus, the P. lateralis is the only one 

 to which it bears the slightest resemblance, and from it, it is distinguished by being proportionably wider, by 

 its numerous longitudinal striaa, and the minuteness of the concentric wrinkles; it is also much smaller. 



