R. M. Brydone — The Origin of Flint. 401 



EXPLANATION OF PLATE VII. 



Fig. L — Analysi? of ambulacrum I of Apalopygus recens, showing Pyrinid and 

 Discoidiid plating below the ambitus, and dissimilarity of the petaloid 

 pores. X c. 4. 



Fro. 2. — Ambulacrum III of the same specimen, similarly enlarged. Pyrinid 

 plating extends into the distal part of the petal. 



Fig. 3. — Similar analysis of ambulacrum I of Oligopodia epiaonus. An 

 extremely simple phyllodc is succeeded by " Bothriocidaroid ' plates to 

 the petal, which shows similar pores. This is a perfectly typical Cassiduloid 

 ambulacrum, its phyllodal simplicity being probably due to the smallness of 

 the species. x c. 6. 



Fig. 4 — Ambulacrum III of the same specimen, similarly enlarged. Both 

 areas have exactly comparable structure, and petals of precisely the same 

 length. 



(In these four figures the ambitus is indicated bj" small dashes.) 



Fia. i>, — Part of corona of Echinolampas depressa at 12"7 mm. diameter. 

 (Modified from A Agassiz, Revision, pi. xvi, fig. IL) The interambulacral 

 tuberculation is roughly like that of Conuhis, but shows greater primitive- 

 ness in the scrobicular rings of granules. The ambulacra! plates are grouped 

 (or perhaps combined) into triads. The probable course of the sutures 

 separating the individual plates is suggested by dotted lines (not present 

 in the original figure). The triadic grouping shown (whatever its actual 

 form) is clearly vestigial, since it is absent in adult forms. The pore-pairs 

 seem in process of becoming single by loss of the outer member. 



The Origin of Flint. 



By R. M. Beydone, F.G.S. 



MR. W. A. RICHARDSON'S paper in the December, 1919, 

 number of this Magazine has furnished a very useful summary 

 of the problem of the origin of flint, with some very interesting 

 suggestions ; but it cannot be allowed to pass altogether unchallenged. 



It is a j)ity that he did not attempt to clear up the confused 

 terminology of Chalk flint. There are, broadly, three kinds : 

 sej)arate flints in rows, interstratified continuous lines of flint, and 

 contrastratified continuous lines of flint. Such terms as lines, 

 bands, seams, layers, beds have been used indiscriminately of all 

 three kinds. It is very desirable that some distinctive term such 

 as " vein " should be attached to the third class, the continuous 

 flint which at any part of its course breaks across the stratification ; 

 while " tabular " might be restricted to the interstratified continuous 

 flint, and " row ", a term which cannot suggest either vein or tabular 

 flint, employed for lines of separate flints. 



The paper is vitiated by the basic but very dubious assumption 

 that all Chalk flint must have originated at the same time. 

 Is it, in fact, conceivable that the interstratified rows of hollow 

 flints can be all contemporaneous with the veins ? These hollow 

 flints practically always contain some amount of soft, loose 

 powder, with an abnormal proportion of fossils in exceptional 

 preservation, and a large majority have a loose spongiform nucleus. 

 The formation and preservation of this assemblage seem to be 



VOL. LVII.— NO. IX. 26 



