Reviews — Dr. A. W. Howe — Zones of the White Chalk. 311 



coranguinum and M. cortestudinanum, and this lie was only able to 

 accomplish after a detailed study of the genus Micraster, the results 

 of which were published elsewhere. He places the junction, not 

 where Micraster prcecursor and M. cortestudinarium are first found in 

 the descending succession, but where certain special forms of these 

 species come in and are associated with Holaster placenta, Hr planus, 

 and some other fossils. 



In the same way a careful collecting of fossils foot by foot enabled 

 him to fix approximate limits to the zones of M. cortestudinarium and 

 Holaster planus. Here I may remark that Dr. Rowe seems to have 

 been under the impression that the term Chalk Rock had been applied 

 by some previous writer to a certain part of the zone of Hoi. planus 

 at Dover; possibly he misunderstood some expressions in Mr. W. 

 Hill's paper on the Middle Chalk of that place ; in any case Mr. Hill 

 and I agree with him that no true Chalk Rock exists there. 



Dr. Rowe's work in Sussex was fruitful in new results. The zones 

 of Holaster planus, Micraster cortestudinarium, and M. coranguinum 

 were defined and distinguished by means of the essential features of 

 the tests of the Micrasters ; but though it is very interesting to know 

 that this can be done by reliance on the forms of Micraster alone, it 

 is obvious that some other means of delimitation must be adopted 

 inland when a sufiicient number of Micrasters cannot be obtained, 

 and I doubt whether the zones will remain permanently as defined 

 by Dr. Rowe. This, however, is a question for the future. 



With the zone of Marsupites we are on safer ground ; neither 

 Uintacrinus nor Marsupites had previously been found on this coast, 

 in spite of special search for them. Professor Barrois had assigned 

 far too great a thickness to this zone in Sussex, and he had never 

 properly defined it. The actual limits within which Dr. Rowe 

 found the plates of these Crinoids was a thickness of 77^ feet, but 

 above this there is a thickness of 20 feet which he regards as more 

 properly included in this than in the overlying zone, and I have his 

 authority for stating that this 20 feet was omitted by mistake in the 

 tabular measurements on pp. 333 and 336. The total thickness 

 of the zone is therefore 97^ feet, and the exposed portion of the 

 Actinocamax quadratus zone at Seaford Head only 150 feet. 



Finally, in this paper he established the zone of Actinocamax 

 quadratus on a firm basis as one of the chief zones in the English 

 Chalk, showing that it contains a fauna which is distinct from that 

 of the zone of Marsupites below, and from that of Belemnitella 

 mucronata as developed further west. At the same time he pointed 

 out that the prevalent Belemnite is not A. quadratus, but^. Blerceyi, 

 which he has since identified with the A. granulatus of Blainville. 



Dr. Rowe's second instalment was on the Chalk of the Dorset 

 coast, and this paper is illustrated by eight excellent photographs of 

 the cliffs, and two maps on the scale of 6 inches to a mile, drawn by 

 Mr. C. D. Sherborn. The plates show what can be done by means 

 of photographs and key-slips to illustrate the stratigraphical detail 

 of a cliff-face, and a comparison of the text with that of the Survey 

 memoir on the same area shows how much more information can be 

 elicited by a careful collection of fossils from the Chalk. 



