Dr. K. A. Gromvall — Dmiyodon in British Mesozoio Rocks. 205 



B. — Cketaoeous. 



5. D. Boehmi, Stoll.^ This widespread species occurs ia the 

 Senonian of Northern Germany, Denmark, and England, and is also 

 found in the Danian at Faxe, Denmark. 



6. D. costatiis, Gronvv." Common in the Senonian of Denmark, 

 Sweden, and Northern Germany ; also found in France, at Meudon. 

 In the Danian there is found a form that in some points differs from 

 that of the Senonian. 



7. J), intusradiatns, Giimbel.^ Upper Chalk of the Bavarian Alps. 



8. D. Nilssoni, v. Hag.^ This species has a very gi-eat extension 

 vertically as well as liorizontally. It ranges from the Upper Gault 

 of Folkestone to the Danian of Denmark. It occurs abundantly in 

 the Challc of England, Northern France, North- Western Germany, 

 the Northern Alps, Denmark, and Svveden. 



As to the systematic position of the genus Dimyodon as here 

 defined, authors are not quite in agreement, mainly because these 

 fossils occur in a rather imperfect state of preservation, and because 

 the free valve is but rarely found. The cardinal parts of the shell 

 are commonly — as a rule in the young ones — inconspicuous and 

 obliterated, while the muscular impressions, in the Cretaceous 

 forms at least, are almost unknown. The ridge-formed teeth, how- 

 evei", sufficiently prove the hinge mechanism to be isodont and these 

 fossils to be allied to the Spondylidas. 



Some authors ^ have considered J). Nilssoni to belong to the genus 

 Cyclostreon, Eichwald,^ the type of which is the Ostrea ])Hcatnloides 

 of Leymerie.'' According to the rather obscure description, the 

 genus Cyclostreon is characterized by the absence of proper cardinal 

 teeth and muscular impressions, in place of which there is a horse- 

 shoe-shaped impression which serves for the reception of ligament 

 and the junction of the valves. 



This interpretation may be the right one for the original species, 

 0. plicatidoides, Leym. ; but for the fossils here mentioned I may 

 deny the possibility of assigning them to the genus Cyclostreon as 

 defined by Eichwald. Frauscher ^ has in the genus Cyclostreon 

 associated some most heterogeneous species, among which two species 

 of Dimyodon, I), intusstriatus, Emmr., and D. intusradiatns, Giimb., 

 are to be found. 



The Cretaceous species of the genus Dimyodon form a well 

 circumscribed and strictly defined group, and, so far as I know the 

 Jurassic species, it seems to me that the same will apply to them. 



1 Stolley: loc. cit. Gronwall: loc. cit., fig. 8. 



^ Gromvall: loc. cit., figs. 4-7. 



^ PaliTeontographica, vol. xxxviii (1891-2), p. 88, pi. iv, figs. 2a-e. 



* Gronwall: loc. cit., figs. 1-3. "Woods: loc. cit. 



* Vogel, 1895 : in Sammlungen d. geol. Eeiclismuseum zu Leiden, neue Folge, 

 herausgegeben v. K. Martin, vol. ii, tome 1, p. 14, pi. i, figs. 4-7. "Wegner : 

 Zeitsch. d. deutsch. geol. Gesellsch. Jahrg., 1905, p. 175. 



^ Lethfea rossica, vol. ii, pp. 406-7. 



' Mem. Soc. geol. France, ser. ii, tome iv (1851), p. 195, pi. ix, figs. I7a-e. 



8 Denkschr. k. Akad. Wiss., INIath. Nat. CI., vol. Ii ; Wien, 1886. 



