290 CARBONIFEROUS LAMELLIBRANCHIATA. 



correctly placed. On the other hand, I regard a large number of the species as 

 synonymous, either representing individuals in different stages of growth, or being 

 founded on merely small varietal characters which are not of specific value. 



I have mentioned the fact above, p. 255, that many specimens of E. Kickxiana, 

 in the fine series possessed by the Royal Natural History Museum of Brussels, 

 show the rudiments of a cardinal tooth, which is so characteristic of the genus 

 Scaldia, de Ryckholt. This first appears as a little irregularity of the surface of 

 the lamellar ridge ; in other specimens there is a distinct rounded nodule, with a 

 depression, for the corresponding tooth of the opposite valve. 



The genus Edmondia is known from Devonian rocks ; but Scaldia, according 

 to de Koninck, at present has not been recognised below the Carboniferous series. 



With regard to the function of the shelly process, the ossicle, which in 

 Edmondia occupies the cavity of the umbo, it probably maintained the shells in 

 contact, acting as a fulcrum for the attachment of either an internal cartilage or 

 some special muscles. An edentulous hinge and shallow muscle-scars show that 

 some special apparatus was necessary, which probably could not, owing to the 

 great thinness of the shells in this genus, be satisfactorily placed on the surface 

 of the valve. 



The genus Edmondia appears to be subdivided into two well-marked groups ; 

 (a) those with fine regular concentric lines of growth, and (b) those with well- 

 marked concentric ridges and sulci. E. unioniformis may be regarded as the 

 type of the former, E. sulcata as characteristic of the latter. I have been for some 

 time undecided whether or no to subdivide the genus on these lines; but it seems 

 to me that intermediate forms exist, e. g. E. rudis, which connects the two groups. 

 Each group comprises suborbicular forms. Moreover all the species which I 

 have included within the genus possess the peculiar process from the back of the 

 hinge-plate which I have termed the ossicle, and which King and previous authors 

 called " cartilage plaques." It appears that the transverse and sulcated shells had 

 this characteristic feature more highly developed and differentiated than the sub- 

 orbicular forms, and consequently, as de Koninck had made his observations on 

 the hinge-plates chiefly on the species belonging to the latter group, his descrip- 

 tions obviously are hardly broad enough to apply to the group of sulcated species. 

 It is due to the perspicacity of Professor King that he perceived that the genus 

 Edmondia really contained such diverse forms as E. unioniformis and E. sulcata. 

 In M'Coy's earlier work this genus, like many others, was misunderstood ; but in 

 his later work (op. supra cit.) he seems to have recognised that the genus included 

 forms with very diverse external characters, and of the nine species described by 

 him as coming from Carboniferous rocks I am able to retain eight in the genus ; 

 and the other, E. E<jertoni, belongs to the family Edmondidaj, but to another 

 genus, closely allied, however, to Edmondia. 



