27G Geological Society. 



I. costellatus Woods * (Chalk Rock) appears to have come from an 

 early form of /. lamarcki by the axis of growth becoming more 

 oblique to the hinge and by the development of a more pointed 

 umbo. 



/. subearcUssoides Schliit., /. digitatus Sow., /. pinnifomiis'^'iW., 

 and /. corrugatus, Woods,^ are allied one to the other, and appear 

 to have arisen from the less convex forms of /. lamarcki by the 

 development of radial folds such as occur occasionally in /. lamarcki 

 and allied species. 



B. /. erippsi Mant. (Upper Greensand to zone of H. suhglobosus) 

 agrees in many respects with the form of /. anglicus which has 

 fewer and less regular ribs, but in this species the hinge has become 

 somewhat shorter, the postero-dorsal part of the shell less com- 

 pressed, the anterior area smaller, the ribs fewer and more irregular 

 with a less strongly-marked posterior curvature. 



In /. erippsi var. reacliensis Eth. (Chalk Marl to zone of //. suh- 

 globosus) the shell has become relatively higher and the posterior 

 curvature of the ribs greater : this variety resembles closely the 

 small forms of /. lahiatus (Schloth.) (zones of Eh. cuvieri and T. lata) ; 

 but in later stages the shell in that species grows extensively in a 

 direction oblique to the hinge, and thus acquires a mytiliform shape. 

 In the zone of TerehratuUna lata the specimens of /. lahiatus are 

 usually longer and less high than in the zone of lili. cuvieri, and these 

 pass into /. lahiatus var. latus Sow, {H. -planus Zone). 



/, inconstans Woods ^ (zone of H. planus to zone of B. mucronata) 

 undoubtedly belongs to the lahiatus stock, and the nearly flat forms 

 approach closely /. lahiatus var. latus, from which they have 

 probably been derived. 



/. halticus Bobm (zone of Marsvpites to zone of B. mucronata) 

 possesses many of the characters of /. inco^tstans, from which it has 

 developed by an increase in the length of the hinge. 



/. lingua Goldf. (zones of Marsupites and A. quadratus) is similar 

 to the nearly flat forms of /. inconstans, but the shell has become 

 liighcr. /. lohatus Goldf, (zones of Marsupites and A. quadratus) \% 

 near to /. lingua, but has developed an angular ridge between the 

 umbo and tlie postcro-ventral extremity. /. cardissoides Goldf, 

 (zone of A. quadratus) and /. tuherculatiis Woods ^ (zone of A. quad- 

 ratus) are closely related to /. lohatus. 



I. undulato-plicatus Eom. (Upper Chalks resembles the nearly 

 flat forms of /. inconstans, from which it appears to have been 

 derived by the development of diverging radial folds. 



> Q. J. Q. ». vol. liii (1897) pi. xxvii, figs. 14-17. 



2 Type, British Museum, L 22528. 



3 G. A. Mantell. ' Foss. S. Downs' 1822, pi. xxvii, fig. 9, & pi. xxviii, fig. 3 ; 

 and F. Dixon, ' Geology of Sussex ' ISoO, pi. xxviii, fig. 29. 



* Type in Dr. A. W. Rowe's collection, from Sowerby (Yorkshire), 



