SPONDYLUS. 133 



hrightomensis by Mantell, occur chiefly in this zone, but occasionally in the 

 previous zone (PL XXIV, fig. 3). 



Marsnpites-zone. — Two types occur : (i) with few or no intermediate ribs and the 

 valves of nearly equal convexity (PI. XXIV, figs. 4, 6); (ii) with many inter- 

 mediate ribs of nearly the same size as the main ribs, and valves of nearly equal 

 convexity : this is not so common as the first type (PI. XXIV, fig. 5). 



A. quadratus-zone. — The same two types occur. Only a few examples have 

 been seen, the average size being apparently less (PI. XXIV, fig. 7). 



B. mucronata-zone. — No intermediate ribs in the examples seen (similar to 

 PI. XXIV, fig. 4). 



Trimingham Chalk. — Two rather small examples are in Mr. A. C. Savin's 

 collection, one without intermediate ribs, and the other having four. Another 

 specimen is in Mr. R. M. Brydone's collection. 



Types. — Sowerby's types from Brighton and Northfleet are in the British 

 Museum. I have not seen the specimen figured by Mantell as Plagiostoma spinosa, 

 nor the type of P. hrightoniensis. S. superhus, Willett, MS., is in the Brighton 

 Museum. The types of 8. duplicatus, Goldfuss, came from the Senonian of 

 Quedlinburg and Coesfeld. 



Distribution. — S. spinosus ranges from the zone of BhyncJionella Gurieri to zone 

 of B. mucronata. A few of the localities are given below. 



i. Zone of B. Cuvieri. — Dover. 



ii. Zone of Terebratulina gracilis. — Dowlands (near Rousdon). AVhitecliff 

 (Seaton). Hooken (near Beer Head). St. Giles's Hill and Twyford Down (Win- 

 chester). East Knoyle (Wilts). The Sussex coast. Dover. Whyteleaf (War- 

 lingham). Dunton Green. Preston (Hitchin). Luton. 



iii. Zone of Holaster j^lanus. — Chapel Rock (Pinliay). St. Giles's Hill (Win- 

 chester). The Sussex coast. Dover. Cuxton. Borstal. Cheveley (Newmarket). 

 Three quarters of a mile north-west of West Wratting (Cambs). 



iv. Chalk Rock. — West Wycombe. Princes Risborough. Cuckhamsley. Box- 

 moor. Luton. Clothall (Herts). Quickwood (Herts). Reed and Newsells 

 (Royston). Underwood Hall (Dullingham). Barkway. Great Chesterford. 



V. Zone of Micraster cor-testudinarium. — West of Beer Harbour. North of 

 Alton Line Junction. The Sussex coast. Dover. Chatham. Purley. Balsham. 



vi. Zone of Micraster cor-anguinum. — Hungry Down, Blandford. Mitcheldever 

 (upper part of zone). Eaton Lane and Winnal Road (Winchester). The Sussex 

 coast. St. Margaret's. Thanet. Gravesend. 



vii. Zone of Marsnpites festndinarius. — Witherington, near Salislniry (lower 

 part of zone). Devizes Road, near Salisbury {Uintacrinus-hsind). The Sussex and 

 Thanet coasts. Margate. 



viii. Zone of Actinocamax quadratus. — The Dorset coast. East and West 



