240 Mr. J. Brown on some Microscopic Fossils 



Trigonia subglobosa, Mor. & Lye. 



Trigonia subglobosa, Morris & Lycett, Gr, Ool. Monog, Bivalves, 

 1853, tab. 5. fig. 21. p. 55. 



Shell nearly circular, convex ; umbones recurved ; area small, 

 with three tuberculated carinse; costse large, closely arranged, 

 angulated, their posterior portions forming a few large perpen- 

 dicular varices. 



It occurs in the freestone beds near Nailsworth; also in the 

 Great Oolite : in both formations it is somewhat rare. 



XXIV. — Note on the Artesian Well at Colchester ; and Remarks 

 on some of the Microscopic Fossils'^ from the Colchester Chalk. 

 By John Brown, Esq., F.G.S., of Stanway. 



[With two Plates.] 



In 1852 an Artesian well was made at the western end of Col- 

 chester, Essex, on the premises of the Old Water Works. The 

 surface of the ground at this spot is about 60 feet below the top 

 of the adjoining hill, known as Balkern Hill. The red sandy 

 gravelf composing the chief mass of Balkern Hill has a total 

 thickness of about 60 feet, but at the well the gravel is about 

 6 feet thick only, with 6 feet thickness of alluvium covering it. 

 The boring then traversed the " London clay,^' 100 feet thick, 

 and the " Plastic clay " (" Reading and Woolwich Series " of 

 Mr. Prestvvich), which is 30 feet thick, and then penetrated the 

 " Chalk with flints " to a depth of 152 feet — making altogether 

 a depth of 294 feet. 



From the chalk brought up by the borer, numerous minute 

 fossils have been obtained by carefully washing the fragmentary 

 chalk and examining it under a lens. These consist of small 

 Serpulae, ossicles of Apiocrinites, and remains of Amorphozoa, 

 Foraminiferse, Bryozoa, and Entomostraca ; such as are usually 

 plentiful in and characteristic of the chalk. 



Figures 8, 9, & 10 in Plate IX. are the calcified remains of 

 three minute species of Sponges, of the genera Tragos and Manon ; 

 but they are too much water-worn and mineralized for identi- 

 fication. 



Of the Foraminiferse, Nodosaria Zippei,N. limbata, Marginulina 



* Mr. Rupert Jones has kindly assisted me in drawing up the following 

 notes on the microscopic fossils of the Chalk. 



t Some remarks on the Gravels of this district, and on their lithological 

 characters, by the author, may be found in the 'Mag. Nat. Hist.' 1835, 

 vol. viii., and in the 'Quarterly Journal of the Geological Society,' 1852, 

 vol. viii. 



