112 CllETACKOUS LAMELLIBRANCHIA. 



Measurements : 



(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) 



Aiitero-veiitral margin 47 45 42 40 38 35 35 84 33 30 26 26 25 24 22 mm. 

 Hinge-margin . . 20 22 24 20 20 20 16 20 18 16 13 12 15 15 12 mm. 

 Diameter from the 

 middle of the carina 

 (perpendicular) to 



posterior margin . 16 19 19 18 18 17 14 17 15 13 12 11 13 — 10 mm. 

 Thickness . . . 17 21 18 15 16 19 15 16 15 13 12 12 11-5 10 11 mm. 



1, 3, 4, 5, 7, 8, 10, 14 from the Crackers, Atherfield. 

 2 ,, Perna-hed, Atherfield. 

 6, 9, 11, 12, 13, 15 from Blackdown. 



Affinities. — The presence of an umbonal plate shows that this species should be 

 referred to the DreissensiidiB. In the few specimens showing the interior I have 

 found no trace of the anterior myophore (for the byssal muscle) which distinguishes 

 Gongeria from DreiHsensia ' — but further specimens to confirm the absence of this 

 structure are desirable. From the Upper Eocene of the Paris basin, associated 

 with marine fossils, M. Cossmann ^ has already described two species of Dreissensia, 

 and that author confirms my reference of this Cretaceous species to the 

 Dreissensiidse. 



This species may be allied to Mytilus tornacensis, d'Archiac, from the Tourtia 

 of Tournay, of which I have seen one specimen, sent me by M. Piret (see p. 91). 

 That species, however, appears to differ from ours in having the carina at 

 a greater distance from the antero -ventral border, also in the presence of more 

 distinct and regular growth-lines with ridges or grooves at right angles to 

 them, the ridges being best marked on the antero -ventral face of the shell. I do 

 not know whether it possesses an umbonal plate or not. M. siibfalcatus, d'Orbigny, 

 from the Cenomanian, appears to have the concentric ribs more distinct on the 

 carina than is usual in M. lanceolatas, but since this feature is seen in some 

 specimens of the latter, and is generally present on the earlier parts of the shell, it 

 can hardly be regarded as distinctive. 



D'Orbigny considered that his Mytilny. Galliennei^ which in outline is similar to 

 the larger examples of M. laiiceolatiis, was identical with M. tornacensis, d'Arch. 



The forms from the Aachen Grreensand described by Miiller as M. tegulatus, 

 Miill., M. lanceolatiis, Sow., and M. falcatiis, d'Orb., whilst referred l)y Holzapfel* 



1 P. Oppenheim, 'Zeitschr. d. deutsch. geol. Gesellsch.,' vol. li (1891), p. 923, pi. li. 



•^ ' Cat. 111. Coq. Foss. Eoc. de Paris,' fascic. 2 (1887), p. 151. 



■^ 'Pal. Fran9. Terr. Cret.,' vol. iii (1844), p. 273, pi. cccxxxix, figs. 1, 2 ; ' Prodr. de Pal.,' vol. ii 

 (1850), p. 165. 



* "Die MoUusk. Aacheu. Kreide " (' Palseontographica,' vol. xxxv, 1889), p. 218, pi. xxv, figs. 

 1 — 9. It should be uoted that these figures are enlarged twice. 



