Upper Chalk 35 



in fossils. The best collecting grounds are Reed, Barkway 

 and Barley, south of the county boundary, but Westley 

 Waterless, Underwood Hall, Wood Ditton, and especially 

 Cheveley, in the east of the county, are also good. The fauna 

 at all these places is large, and is especially notable for its 

 aberrant ammonites and gasteropods. Scaphites Geinitzi and 

 Heteroceras are characteristic. Pleurotomaria, Turbo, Trochus 

 and Natica, Rhynchondla plicatilis, Terebratula carnea and 

 several species of Micraster abound. Fossils are generally 

 very difficult to extract, and weathered specimens are often 

 the best. 



The beds above the Chalk Rock yield far more specimens 

 of Micraster than of Holaster, and accordingly were placed 

 by the Geological Surveyors in the zone of Micraster corbovis, 

 which has since been correlated with the zone of Micraster 

 cortestinudinarium. Recently, however, specimens of the 

 Micrasters have been sent to Dr Rowe and have been recog- 

 nized by him as belonging to the l Planus zone,' so we now 

 have to include a considerable thickness of this higher chalk 

 in the zone of Holaster planus. 



The area occupied by the surface outcrop of these beds 

 within the county is quite small, for they are almost entirely 

 covered by Boulder-clay. Fine exposures, however, do occur, 

 and from many of them most beautiful specimens of the 

 various fossils may be collected. The best localities are 

 Balsham, a pit north-west of West Wratting, and the Middle 

 of the World near Horseheath. 



The most accessible is the Balsham lime pit. There most 

 beautiful examples of Spondylus spinosus with all its spines 

 are abundant, and uncrushed examples of Micraster, Rhyn- 

 chonella plicatilis and Terebratula carnea occur. Occasional 

 giant specimens of Holaster planus have been found. 



The West Wratting pit is very rich in Micrasters but 

 other fossils are rather rare. 



The highest members of the chalk series of Cambridge- 

 shire probably belong to the Micraster cortestinudinarium or 



32 



