215 



" A well marked and very pretty species, and the earliest known in Salter s 

 British rocks. It is hardly ever more than one third of an inch wide, descri i' tion - 

 and most specimens are not more than half of that size. The length is 

 less than the breadth in proportion as seven to nine. Our figures repre- 

 sent the shell as distorted in various positions, but the above is about 

 the average measurement. Both valves are somewhat convex, but the 

 dorsal valve has a broad central depression of a triangular shape, bounded 

 by two rather prominent ribs, out of the ten or twelve strong ones that 

 radiate from the beak, and the sinus is occupied by two sub-central and 

 very distinct ones. The lateral ribs are strongly interlined by others 

 half way up, the intervening ribs becoming as strong as the primary ones, 

 and these again by smaller and shorter ones in the intervals. All are 

 crossed by strong, and interrupted, but rather wavy ridges of growth, so 

 as to decussate the surface in rather a remarkable way. The other valve 

 is alike in sculpture, but has a rather prominent beak. The number of 

 ribs varies greatly, but not in a way to make us believe we have more 

 than one species. Sufficient differences are not seen in the Scandinavian 

 specimens to warrant us in separating these which occur in abundance 

 in the alum slates in limestone layers. 



The teeth diverge slightly in both valves ; in the dorsal valve they are 

 subparallel, and short, or even curved a little inward, and are not thick- 

 ened ; while the cardinal process between them is a mere line or thin 

 edge, which extends as far down as the length of the short lateral teeth 

 but is often very obscure, and sometimes seems to be altogether absent. 



There is little doubt that this is Dalman's species from Egeberg, 

 though the specimens we have from thence have less prominent ribs, and 

 a generally smoother appearance." 



If the reader will compare together these several descriptions of this 

 species by the above writers, he will be surprised at the diversity they 

 exhibit. This diversity, it appears to the writer, is due partly to the 

 imperfection of the descriptions of the early writers, but chiefly to the 

 remarkable variability of the species. 



Wahlenberg takes no notice of the difference between the dorsal and 

 ventral valves, for he ascribes to them an equal convexity, and say that 

 the two valves occur together ; but Dr Lindstrom intimates that they 

 have never been found united, and from specimens which he has kindly 

 sent me, it is clear that the Swedish variety has the usual sulcus in the 

 dorsal valve, though in some cases only faintly marked. In the typical 

 form of this species the cardinal area is short and the beak quite low ; and 

 as the convexity of the two valves does not differ greatly, this appears to 

 be the form which Wahlenberg had in view when describing the species. 



