SEPTTFER. 109 



Modiola quadra ta, Sowerby, from the Upper Chalk, is, I believe, only a 

 somewhat crushed example of this species, similar to the one figured on our 

 PI. XVIII, fig. 12. 



Holzapfel considers that the forms from the Aachen Greensand, originally 

 described by Miiller as Mi/fihis lineatus and M. scalaris, should be referred to M. 

 lineafiis, Sowerby. The Aachen species is very variable, and T am greatly indel)ted 

 to Prof. Holzapfel for the loan of six specimens which vary in their greatest length 

 from 0-5 to 15 mm.^ The Aachen shell is clearly distinct from our species ; it is 

 more curved, more irregular, smaller, and Avith the radial ril)s more strongly 

 marked and not crossed by concentric ribs, so that the cancellate appearance is not 

 seen. 



The form figured as Mijtiliis ciphjanus by de Ryckholt,^ from Ciply and 

 Maestricht, is perhaps a small example of S. lineatus (Sow.). 



M. ssquat oralis, Mayer-Eymar,^ from Somaliland, is perhaps an allied form, but 

 is imperfectly known at present. 



Bemarls.— The variations in this species consist chiefly in the amount of 

 inflation and curvature of the valves, and in the flattening of the antero-ventral 

 surface. 



The examples from the Hythe Beds are usually larger, rather more inflated, 

 and have the antero-ventral surface more flattened than those found in the Upper 

 G-reensand and Cenomanian. But all the different forms which the shell takes may 

 be seen at all horizons, and from the examination of a large series of specimens I 

 am convinced that the Lower Cretaceous forms cannot be separated from those of 

 the Upper Cretaceous. The relative proportions of the valves of course change as 

 age increases. 



On account of the thinness of the shell the form of the valves has often become 

 somewhat altered by pressure, and the shell itself — especially in specimens from the 

 Hythe Beds — has often partly disappeared. 



None of the specimens I have seen show the interior of the valves, but some 

 of the internal casts give indications of the presence of an umbonal plate. 



The name M. Cottx, Romer, is adopted by Geinitz (1873) for this species, and 

 M. Gavieri, Matheron, by Pictet and Campiche (18G7), since the name Mytilits 

 lineatus had been used by Gmelin in 1789 for another form, and Modiola angusta. 

 was also preoccupied by Deshayes (1824). D'Orbigny in 1850 substituted the 

 name siihlineatus for lineatus, Sow. If Sowerby's species be referred to Septifer, 

 then his specific name may be retained. 



^ It should be noticed that Holzapfel' s figures of this species are enlarged three times. 

 - "Melanges Paleont.," pt. 1 (' Mem. cour. et Mem. des Sav. etrangers,' vol. xxiv, 1852), p. 152, 

 pi. ix, figs. 12, 13. 



^ ' Vierteljahrs. nat. Gesellsch. Ziirich,' vol. xxxviii (1893), p. 254, pi. i, figs. 7, 8. 



