218 



MOLLUSCA FROM THE CRAG. 



regularity of growth. The principal difference between this and G.fragilis is a larger 

 posterior side, which is biangulated, the posterior dorsal portion not sloping off to a 

 point as in the recent shell : the laminae in our fossil are also larger, more remote, and 

 the palleal sinus extends only to a line drawn perpendicularly from the umbo ; in 

 G.fragilis it projects beyond. The right valve of our shell has two large diverging teeth, 

 while the left one possesses one large central, triangular, bifid tooth, with a cavity on 

 each side, and two small rudimentary teeth ; a large fulcrum for the ligament ; the 

 laminae are prominent, and erect, not reflexed. 



Tellina fragilis, Linn., is found fossil in the Sicilian Beds, as, also, quoted in the 

 Faluns of Touraine ; but I have not seen it from the Crag. 



Don ax,* Linnaus. 1758. 



Tellina. Adans, 1757. 



Chion. Scojioli, 1777. 



Cuneus. Da Costa, 1778. 



Serrula. Chemn. 1782. 



Capisteria. Gevers, 1787. Sec. Gray. 



PERONiEA, PeRON^ODERMA (sp.) Poll, 1791. 



Latona. — Schum. 1817. 

 Hecuba. Id. 

 Ipiiigenia? Id. 

 Capsa. Lam. 1818. 

 Donacina. Ferus. 1821. 

 Egeria? (sp.) Lea. 1833. 



Generic Character. Shell transverse, inequilateral, equivalved, more or less wedge- 

 shaped ; posterior side the shorter ; surface generally smooth and glossy, sometimes 

 finely striated or decussated ; covered by an epidermis in the recent state. Margin 

 plain or crenulated. Hinge composed of two cardinal teeth in one valve, and one in 

 the other, with more or less developed lateral teeth. Muscular impressions ovate with 

 a large and deeply indented mantle-mark. Ligament external. 



Animal somewhat oblong, the mantle open in front, with fringed or partially fringed 

 margins. Siphons not very long, separated their entire length ; foot large, sharp 

 edged, and pointed. 



The principal distinction between this genus and that of Tellina is the truncation 

 of the posterior side, and the general wedge-shaped form of the shell, as also the 

 absence of the flexous fold in the margin, and it is in general more inequilateral, but 

 the two genera are very closely allied. Tellina is made the type of a family by the 

 malacologists, and the present genus is alike honoured with a similar distinction, upon 

 what grounds they are so widely separated I am unable to discover. 



A fossil from the Carboniferous Period is described under this name, but its true 

 generic position is doubtful, and there is no certainty of its existence in the fossil state 

 anterior to the older Tertiaries. 



* Etym. hoval, a sea-fish. Pliny. 



