114 CRETACEOUS LAMELLIBRANCHIA. 
that whilst a number of living genera of fresh-water mollusks occur in pre-Tertiary 
rocks, Dreissensia has not yet been found associated with them.' 
Remarks.—Four examples from the Blackdown Greensand were described 
by Sowerby as distinct species under the names edentulus, lancevlatus, tridens, 
prelongus. Most later authors (d’Orbigny, Forbes, Bronn, Pictet and Renevier, 
Pictet and Campiche, Briart and Cornet, Stoliczka, Whiteaves, etc.) have considered 
these forms to be mseparable, and have united them under the name lanceolatus, 
but that view was not shared by Morris. An examination of all the available 
specimens leads me to agree with the opinion generally held. 
In 1850 d@Orbigny regarded the Lower Cretaceous examples as distinct from 
those found m the Blackdown Greensand, and named them Mytilus abruptus. 
Pictet and Campiche did not uphold this separation. The only difference that I can 
detect is that, on the average, the examples found in the earlier beds reach a larger 
size than those in the later. 
T'ypes.—From the Blackdown Greensand: M. edentulus is m the British 
Museum; M. tridens and M. prelongus are in the Bristol Museum. I have not 
been able to trace the type of M. lanceolatus. 
Distribution. —Perna-bed, Crackers, and Fitton’s Beds 32 and 45, of Atherfield. 
Perna-bed of Sandown. Atherfield Beds of East Shalford and Peasmarsh. 
Ferruginous Sands of Shanklin. Sandgate Beds of Parham Park. Blackdown 
Greensand (zones x and xv). Greensand of Haldon. Upper Greensand of 
Shaftesbury. 
Family—MODIOLOPSIDA, Fischer. 
Genus—Myoconcua, J. de OC. Sowerby, 1824. 
(Min. Conch., vol. v, p. 103, pl. cecelxvii.) 
Myoconona creracka, A. @Orbigny, 1844. Plate XX, figs. 3a, b. 
21832. Myrinus srmpiex, A. Passy. Géol. de la Seine-infér., p. 6 (expl. of plates), 
pl. xiii, figs. 4, 5. (Non M. simplex, 
Defrance, 1824.) 
1 W. J. Sollas, “On the Origin of Fresh-water Faunas,” ‘Scient. Trans. Roy. Dublin Soc.,’ 
ser. 2, vol. iii (1884), p. 106; C. A. White, ‘Third Aun. Rep. U.S. Geol. Survey’ (1883), p. 428. It 
has been suggested that Mytilus membranaceus, Dunker, from the North German Wealden of Obern- 
kirchen, Egestorf, Oesede, etc., and the Purbeck beds of Nienstedt and Linden, may belong to the 
Dreissensiide, but the characters of the interior of the shell are at present unknown; the same may 
be said of Mytilus Lyelli, Sowerby, from the English Purbeck and Wealden. See Dunker, ‘ Mon. 
Norddeutsch. Weald.’ (1846), p. 25, pl. xi, f. 10, 11; C. Struckmann, ‘ Die Wealden-Bildungen von 
Hannover’ (1880), p. 68, pl. i, f. 11, 12; P. Oppenheim, ‘Zeitschr. der deutsch. geol. Gesellsch..,’ 
vol. xliii (1891), p. 944. 
