ARCIDvE. 125 



Dimensions. — PI. IV, fig. 14, measures — 



Antero-posteriorly . . . .32 mm. 



Dorso-ventrally . . . .19 mm. 



From side to side . . . .14 mm. 



Localities. — England : The Carboniferous Limestone of Hill Bolton, Yorkshire, 

 and Thorpe Cloud, Derbyshire. 



Observations. — This species was one of the first of the genus described by 

 de Koninck, and it has been freely reported from nearly every British locality 

 of Carboniferous Limestone; but at the time of arranging the plates I was able 

 to obtain only two specimens (both in the collection of the Geological Survey at 

 Jermyn Street) which belonged to the species in question. Since then I have 

 obtained several specimens from the limestone of Hill Bolton, near Grassington, 

 and from Thorpe Cloud, Dovedale, in some of which the details of the hinge and 

 interior are finely shown. I regret that they were found too late to be drawn on 

 the plates for this Part. I have noted, in my remarks on M. peralata, the dis- 

 tinguishing characters between these two forms. I believe that most of the forms 

 reported in various lists as belonging to M. lamellosa should be more correctly 

 referred to M. Verneuilii or M. Flemingi. This species appears to be a connecting 

 link between the two forms, being flatter than M. Flemingi and more gibbose than 

 M. Verneuilii. The specimen figured on PI. IV, fig. 14, has on its reverse side a 

 portion of the hinge-plate visible, and shows also the pallial line. 



I find that the specimen of this species figured by de Koninck in his first 

 work (1842) differs essentially in shape from that given in his later publication 

 (1885), the former resembling very strongly the series I figure on PI. V, figs. 15 

 to 19, of Posidoniella elongata. I think it not at all unlikely that some of the 

 other species figured by de Koninck, e. g. M. amoena and M. bursa, may be shown, 

 when a larger number of specimens have been collected, to be merely varietal 

 examples of M. lamellosa. De Koninck himself recognised in his later work (op. 

 supra cit.) that one of the original species of this genus, M. virgula, was only a 

 variety of M. lamellosa. 



Family ARCID^E. 

 Genus Parallelodon, De Koninck, 1885. 



Synonyms : 



Aecites, Martin, 1809. Petrifact. Derbiensia, pi. xliv, fig. 7. 

 Aeca, Sowerly, 1825. Min. Conch., vol. v, p. 115, pi. cccclxxiii, fig. 2. 

 — Fleming, 1828. Hist. Brit. Anim., p. 399. 



