Floor of the English Channel. 65 



number of small shell fragments, which always include some of 

 Echiuoderm plates and ossicles, as well as some Inoceramus prisms. 

 There are also some perfect but small Foraminifera, such as 

 Textularia, Rotalia, and small Glohigerina ; and a variable number of 

 sponge spicules replaced by calcite and of cavities which have been 

 occupied by spicules. Here and there larger pieces of shell or 

 Ecbinoderm plates are to be seen. 



They differ slightly from one another in sucb a manner as different 

 pieces of chalk from the same zone might be expected to vary. Thus 

 21c and 26« have very few Inoceramus prisms and not many sponge 

 spicules, while 72J is full of spicules and there are few prisms ; 

 26fl; is also notable for containing a large piece of Echinid plate and 

 a fragment of composite Polyzoan zoaria. 



These slides agree very closely with one cut from a bed of hard 

 rocky chalk in the middle of the zone of JE[. planus at Cruxtou in 

 Dorset, the No. 3 of the section given in Cretaceous Rocks of Britain, 

 vol. iii, p. 119. This rock has the usual variety of contents, but 

 most of them are small, and neither Inoceramus prisms nor sponge 

 spicules are numerous. Further, there is no glauconite, so that its 

 composition is really just the same as that of the 2\c and 26« pebbles. 



They also have much resemblance to the material of a piece of the 

 planus zone obtained by Mr. A. C. G. Can:eron from Annis Knob, near 

 £eer. In the slide prepared from this the shell fragments are all 

 small, except one large piece of Echinid plate, but sponge spicules 

 are abundant, Inoceramus prisms fairly numerous, the perfect 

 Foraminifera small, and there is no glauconite ; so that this compares 

 very closely with M. 12b, 



Lastly, they are comparable with a slide cut from a piece of hard 

 chalk from the zone of Micr aster cortestudinarium at Pinhay, near Lyme 

 Regis, for which I am indebted to Mr. A. W. Rowe. This is crowded 

 with cells, spheres, small Foraminifera, and small shell fragments, 

 among which bits of Echinoderm shell are numerous ; there are many 

 sponge spicules, but not much Inoceramus debris. One or two small 

 grains of glauconite occur, and the specimen must have come from the 

 base of the zone, the structure being like that of the H. planus zone. 



The slide which differs from the other three is labelled M. 72a. 

 It contains very few spicules and a larger number of Inoceramus 

 prisms, as well as many irregular fragments of shell, the derivation 

 of which it is difficult to recognize. In this respect it resembles two 

 other pebbles which will be mentioned hereafter. 



Pebbles not previously cut. — Six of these have been sliced and 

 mounted by Mr. Hill, but one of them turns out to be of some other 

 kind of limestone, not a Cretaceous chalk, and two others are so much 

 perforated (Nos. 17 and 25) as to be of little use. The following are 

 brief descriptions of the microscopical structure of the five chalk- 

 pebbles. 



ISTo. 58. This is a compact chalk, the matrix of which is nearly as 

 full of recognizable organic fragments as the chalk of the H. planus 

 zone. There are plenty of cells, spheres, and small shell fragments, 

 some of which appear to be broken and eroded Inoceramus prisms, but 

 there is little Echinoderm debris and very few Foraminifera or 



DECADE V. — VOL. X.— NO. II. 5 



