PLICATULA.. 143 



testadinarium of Dover and the Sussex coast. Zone of M. cor-angidnum of the 

 Thanet coast, the North Foreland, St. Margaret's, and the Sussex coast. Uintacrinus 

 band of Kingsgate. Marsvpites zone of the Sussex and Thanet coasts. Zone of 

 B. mucronata of Clarendon, near Salisbury. 



(8) 



(9) 



(10) 



17 



18 



21 mm. 



16 



16 



18 „ 



Plic'Atula sigillina, Woodward, 1864. Plate XXVI, figs. 19—22. 



? 1852. Spondylus dichotomus, A. Biivignier. Statist, gcol., etc., de la Meuse, 



Atlas, p. 25, pi. xix, figs. 16, 17. 

 1864. Plicatula sigillina, S. P. Woodward. Geol. Mag., vol. i, p. 112, pi. v, 



figs. 1—5. 



Description. — Shell small, semi-oval or semicircular in outline, a little oblique. 

 Hinge margin long. Right valve attached by nearly the whole of its surface ; 

 interior with slightly raised radial ribs, somewhat irregular, becoming more 

 numerous at the sharp raised margin; beyond this margin is a broad smooth 

 sloping border bounded by a raised edge, outside which are, in some cases, radial 

 ribs. Left valve slightly convex, ornamented with well-marked concentric lamellre. 



Measurements : 



(1) (2) (3) (4) (5) (6) (7) 



Length . 14 15 15 6-5 11 16 9 

 Height^ . 12 15 17 6 10 12 9 



(1 — 3) M. cor-testudinarium zone, Chatham. 

 (4 — 6) M. cor-anguinum zone, Gravesend. 

 (7 — 10) B. mucronata zone, Hartford Bridge, Norwich. 



Affiiuties. — The form from the Gault of Clermont and Les Islettes (Argonne, 

 Meuse), described by Buvignier as Spondi/lns dichotomus, is probably identical with 

 this species; it is especially like examples of P. sigillina from the Cambridge 

 Greensand and the Cault of Folkestone. 



P. sigillina differs from the other Cretaceous PUcatulse here described in being 

 attached by almost the entire surface of the right valve, and in the absence of 

 radial ribs or folds on the left valve ; in these respects it resembles the completely 

 fixed forms of the recent species P. philippinarum, Hanley. 



Bemarks. — This species occurs attached to EcUnoconjs, Inoceramus, Belemnitella, 

 and other fossils. The fixed valve is common in the Norwich Chalk and the 

 Cambridge Greensand. The left valve is much less frequently found than the 

 right, and at present appears to be known only from the Upper Chalk and the 

 //. planus zone. The inner layer of the shell has undoubtedly disappeared, and 



1 Measured obliquely. 



