FAUNA OF ÏÏÏH ST. JOHN GEOUP. 47 



" Both shells are slightly elevated ; both, however, have a slightly depressed [hollow ?] 

 in the middle opposite each other. The margin is square oval, with sides sloping away, 

 and slightly bending down at the back. The hinge of the ventral [dorsal] shell is 

 straight ; in the dorsal [ventral] valve, on the contrary, the edges of the hinge are bent 

 into a very blunt angle. This is the only way by which one can distinguish the valves 

 from each other, for the area, which in itself is very small, lies always on the under edge. 

 The greatest width is in the middle of the length. From eight to ten radiating lines go 

 from the middle point [umbo] out, and increase at the border to from eighteen to twenty 

 lines. Very fine crowded lines of growth cross these and form a very pretty pattern. 

 Length to the w^idth as 100 to 131. 



" This little mussel is crowded together in enormous numbers. They build alone the 

 alum shales of Andrarum, in Schona. Yes; Dalman relates that such beds occur through- 

 out the whole of West Gottland, and also in sevei'al unnamed provinces of Sweden." 



Of this species Salter gives the following description, referiing it to the genus Orlhis: 



" A well-marked and very pretty species, and the earliest known in British rocks. It 

 is hardly ever more than one-third of an inch wide, and most specimens are not more 

 than half that size. The length is less than the breadth in proportion as seven to nine. 

 Our figures represent 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 subcentral 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 shorter and smaller ones in the intervals. All are crossed by strong, and inter- 

 rupted, but rather wavy ridges of growth, so as to decussate the sui-face in rather a 

 remarkable way. The other valve is like 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 ScandinaA'ian 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 thickened ; 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 Caiman's species from Egeberg, though the speci- 

 mens 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 which they exhibit. This diversity, 

 it appears to the writer, is due partly to the imperfect descriptions of the earlier 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 says that the two valves occur together ; but 

 Dr. Liudstrom intimates that they have never been found united, and from the 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 



