BIHANG TILL K. SV. VET.-AKAD. HAXDL. BAND. 3. N:0 12. 11 



iuequalities ; in weathered stones only can it be obtained tole- 

 rably perfect, but worn and indistinct (as fig. 7) ; the small Car- 

 dinal process seems to be bifid; from under this process there 

 extends a flattened mesial ridge, which iisually soon becomes 

 obliterated; the muscular scars are small and verv obsolete. Sur- 

 face of the valves marked by numerous radiating ribs, intersected 

 by numerous concentric lines of growth, of Avhich, in the adult, 

 one or two are more marked and imbricating. The radiating ribs 

 increase in number through interpolation ; a real bifurcation 1 have 

 not seen in any instance, though a younger rib often begins not 

 in the middle between two older ones, but much nearer to the 

 one of them. The interpolation takes place verv irregularly. 



Full-groAvn specimens vary but little in shape and orna- 

 mentation. Only in the interiör are the features much more 

 marked in some specimens than in others. The young indi- 

 viduals vary more. Fig. 9 represents a ventral valve with one 

 imbricating line near the margin; it is somewhat narrower ante- 

 riorly than the tA'pical specimens. Fig. 10 represents another 

 ventral valve, in which no sucli line has yet been developed. 

 The dorsal valves figs. 11 and 12, in pl. 11, belong, I think, to 

 young examples of this species, though they difler somewhat 

 from the type, being more transversely oval, and less widening 

 anteriorlv; besides, in one of them the cardinal angles are more 

 acute than usual. 



Some adult, typical specimens measured: 



Length. Breadth. 



Dorsal valve 11 mm. lo mm. 



» )' 14 )' 17 )> 



Aentral » 12 )} 14 >• 



Some junior specimens, in which the characters of the adult 

 were not yet developed, measured: 



Lengtli. Breadth. 



Dorsal vahe 9 mm. 12 mm. 



)) » 7 >■> 9,5 )^ 



y> » 8 )> 10 )' 



I do not know any species ^with which this can be con- 

 fbunded. At least the adult specimens are always easily recog- 

 nized. In the young ones the characters are less marked; espe- 

 cially in the dorsal valves. The interiör and the internal cast of 

 the \entral valve is in all stages of orrov/th easilv recosrnized. 



