214 



which occurs in the same rock. Each valve possesses an equal con- 

 vexity, so that when united they very much resemble the seed of a lentile. 

 The valves are seen to have been very thin in their substance. 



Dalman's description is fuller, and is as follows : " A small species 

 with very slender and fragile valves. Length about 4 mm., and of about 

 the same, or somewhat greater breadth. An entire specimen is rarely 

 found, but the species is very easily distinguished on account of the 

 stone in which it occurs, as it has scarcely any other species of Tere- 

 bratulite with it. The shell is suborbicular, with the base somewhat 

 prominent, and a little convex ; toward the margin it is sensibly com- 

 pressed. Striae of undulating lines, in number about twenty, but in- 

 definite. Without a yoke (deltidium) or canal to the dorsal (ventral) 

 valve. In the complete state no transverse furrows have been 

 observed, but deprived of the epidermis (outer shelly layer) the radiating 

 striae are wanting, and the shell appears concentrically striulate. The 

 true structure of the hinge is not rightly explored, but, on account of the 

 external aspect, and its place in the most ancient strata, the species is 

 suspected to belong to the genus A trypa." 



Early Leopold von Buch, in the work above cited, presents us with another 



descriptions of . . ,. . . , , . . . ., ., , , ,. 



this species. view of tn is species, and from bis impressions of its form and relations 



referred it doubtfully to the genus Spirifer. His account is as follows : 



" Both shells are slightly elevated ; both, however, have a slightly 

 depressed (hollow?) in the middle opposite each other. The mar 

 gin it square-oval, with sides sloping away, and slightly bent 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 dis- 

 tinguish 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 (transverse diameter). From 8 to 10 radiating 

 lines go from the middle point (umbo) out, and increase at the border to 

 form 18 to 20 lines. Very fine crowded lines of growth cross these and 

 form a very pretty pattern. Length to the width as 100 to 131. 



This little mussel is crowded together in enormous numbers. They 

 build alone the alum-shales of Andrarum in Schona. Indeed, Dalman 

 says that such beds occur throughout the whole of West Gottland, and 

 also in several other provinces of Sweden." 



Of this species Salter gives the following description referring it to the 

 genus Orthis : 



"Wahlenberg Petr. Tell. Suec. p. 66. 



