TEREBRATELLA. 63 



Diagnosis. Shell inequivalved, somewhat transversely oval, perforated valve, more 

 convex than the other, with a produced beak, truncated by a rather large circular foramen; 

 deltidium obtusely triangular, and in two pieces ; cardinal or false area concave, nearly 

 smooth ; valves costated ; seven or nine round costse imbricated, increasing in size, not in 

 number, towards the margin. The central medio-longitudinal plait larger than the lateral 

 ones ; shell punctuated, marked by numerous lines of growth ; loop short, simply attached 

 to crura, and extending to about half the length of the shell ; dimensions variable. 

 Length 5, breadth 5, depth 3 lines ; but some foreign species measure, length 6, width 8, 

 depth 4 lines. 



05s. This species seems to have been first described by M. Defrance, under the name 

 of T. Jlabellum, but it is better known by that of T. palmetto, to French Geologists ; and was 

 only within a few years noticed in England : it occurs in the Bradford Clay, of Bradford 

 and Corsham, Wilts, where it was found by Messrs. Walton, Pearce, and Waterhouse. In 

 France it is met with in the Oolite of Ranville, at Luc, and Langrune, near Caen, where it 

 attains very large dimensions ; and I am indebted to my friend, M. Deslongchamps, for 

 working out the interior. This species belongs to the Loricata of Von Buch, in which the 

 ribs of the smaller valve envelope those of the larger. Ter. JlabeUum is also remarkable for 

 its rare variability ; it is always easily recognised, being rather a scarce species, especially in 

 England. Figs. 20 and 21 of our plate are enlarged. 



Genus — Terebratella, D' Orb. 1847. 



Diagnosis. Shell inequivalved, oval, sometimes transverse, larger valve more convex 

 than the smaller or imperforated one, which is in many cases flatter ; cardinal edge straight 

 or slightly curved ; a well-defined rather flat hinge area existing in larger valve, beak com- 

 monly straight and truncated by a foramen of an oval or irregularly triangular shape 

 placed more under than above the summit, formed out of a small portion of the substance 

 of the beak ; cardinal area and triangular deltidium in two pieces, which are disunited in 

 many cases, the aperture being completed by a small portion of the umbo. Structure 

 punctuated, surface striated or plaited, often bifurcating ; hinge articulating by means of 

 two teeth in larger and corresponding condyles in smaller valve. Loop generally long, 

 doubly attached, proceeding from the crura, but before attaining its greater length it gives 

 off a flat, wide, more or less horizontal process, likewise attached to a central longitudinal 

 elevated septum, the principal lamella proceeding till it doubles itself in the shape of a 

 loop, as in true terebratuke. 



Obs. We are only acquainted with one British Jurassic species attributable, according 

 to M. D'Orbigny, to this genus, viz. Terebratella hemisphcerica, Sow. 



