Dr. K. A. Qromvall — Dimyodon in British Mesozoic Bocks. 203'' 



struck me that this fossil was closely allied to a group of bivalves, 

 well known to me as occurring in the Chalk, where it is represented 

 by the genus Dimyodon, Munier-Chalraas. 



In 1842 von Hagenow ^ described, under the name Ostrea Nilssoni, 

 a bivalve shell found in the White Clialk of Riigen, attached to 

 several other fossils, e.g., belemnites, oysters, and sea-urchins. 

 Although he gave parallel diagnoses of this shell and Ostrea 

 Jiippopodium, Nilss., he did not see the generic difference between 

 his fossil and the oysters. 



Later on, 1891, J. Bohm - referred Ostrea Nilssoni to the genus 

 Dimyodon, Mun.-Chalm., and in the same year Stolley ^ described 

 a new species, as also did I myself in the year 1900,* then figuring 

 the three Danish species that all occur rather commonly in the' 

 Chalk of North- Western Germany and Denmark, D. Nilssoni, von 

 Hag., D. Bohmi, Stoll., and D. costatus, Gronw. 



This group of fossils also occurs in the British Chalk. D. Nilssoni 

 was in the year 1864 described by S. P. Woodward^ under the 

 name Plicatula sigillina, S. P. Woodw. H. Woods, in his recent 

 monograph of the British Chalk Bivalves," having in the description 

 of 1901 (Pt. iii, p. 143 ; ph xxvi, figs. 19-22) used S. P. Woodward's 

 name, in an addition of 1903 (Pt. v, p. 225) altered it into the 

 current name, Dimyodon Nilssoni. 



When writing the note on Dimyodon in 1900 I found in the 

 collections of the Museum of Copenhagen some British specimens of 

 D. Nilssoni, V. Hag., and also one of D. BoeJimi, Stoll., fixed on 

 a Micrnster cor-testudmarium. 



During a visit to England in the Summer of 1905 I saw in the 

 British Museum, besides several specimens of D. Nilssoni from 

 various localities, one specimen of D. Boelimi, or a closely allied 

 form, fixed on an Echinocorys froin the Upper Chalk of Gravesend, 

 Kent, and of Grays, Essex. 



In the collections in Copenhagen I had also seen some specimens 

 of this genus from the Jurassic fixed on other fossils of greater 

 importance and themselves never determined. In the Geological 

 Museum of Bath some specimens of Dimyodon, growing on a Lima 

 from the Lower Lias, attracted my attention. They were labelled 

 Plicatula intnsstriata, and tlie localities were Shepton (Somerset), 

 Southerndown and Bridgend (Glamorganshire). There was also 

 a smaller species of Dimyodon fixed on a Lima from the Upper 

 Lias. I had no opportunity of an examination, but only surveyed 

 them as they were exhibited in the show-case. 



Later on, in the paper of Mr. Richardson'' I met with the 

 excellent figure of the Plicatula intusstriata, Emmr., clearly proving 



1 Neues Jahrb. f. Min., 18-42, p. 546. 



2 PaliBontographica, vol. xxxviii (1891), p. 89, pi. iv, fig. 7. 



3 Mitth. Min. Inst. Univ. Kiel, vol. i (1891), tome vii, p. 243, pi. vii, fig. 8. 

 * Medd. dansk geol. Foren., No. 6 (1900), pp. 73-80, pi. ii, figs. 1-8. 



5 Geol. Mag.,"Vo1. I (1864), p. 112, PL V, Figs. 1-5. 



^ A j\Ionograph of the Cretaceous Lamellibranchia of England (Palasont. Soc.,. 

 1899-1903). 

 ' Q.J.G.S., vol. Isi (1905), p. 423, pi. xsxiii, fig. 1, 



