22 LINNARSSON, BRACHIOPODA OF THE PARADOXIDES BEDS. 



Acrotreta suhconica and Åcrotreta Nicliolsoni. The surface of 

 the slaell is marked by numerous lines of growth, which are 

 most conspiciious in the false area, över which they continue 

 uninterrupted. Sorae specimens show in the anterior part a 

 very slight indication of radiating ribs. Of the interiör I have 

 not found any examples quite satisfactory. It is smooth and 

 polished; in a few specimens only some radiating lines are 

 very faintly indicated. The forameu is larger than at the 

 surface, and always easily perceptible. Before the foramen 

 there is a semicircuhi,r ridge, judging from the internal casts, 

 which in that place always exhibit a semicircular groove. The 

 false area is also slightly indicated in the interiör and in the 

 internal casts. Of the miiscular scars I have not been able to 

 detect any trace. — The dorsal valve is almost flat. The um- 

 bone is marginal, with two wart-like protuberances as in the 

 ventral valve. From the umbone a verv slight depression is 

 often seen to extend to about the middle of the valve. The 

 surface is rough, with several concentric lines of growth, which 

 cross the posterior margin of the shell. Of the interiör I have 

 only found one single good specimen (fig. 48). As it exhibits 

 some remarkable features, I shall describe it in some detail, 

 though 1 am uncertain whether this description will, in every 

 respect, be generally applicable. Near the umbone there is a 

 small polygonal space, marked with several equidistant ridges 

 and grooves, parallel to the margin of the space. This I 

 cannot interpret otherwise than as the scar of an umbonal 

 mascle, corresponding to that of Lingula. The umbope itself 

 is in this specimen somewhat mutilated, but from another spe- 

 cimen it appears that there are two extremely minute pits, 

 corresponding to the wart-like protuberances on the external 

 surface of the shell. From the anterior part of the wrinkled 

 polygonal space a Ioav, dome-shaped, longitudinal ridge extends 

 to about the middle of the valve, where it sends oiF, on both 

 sides, some very faint linear ribs. The longitudinal ridge se- 

 parates two pair of muscular scars, which are shallow and only 

 feebly marked. The posterior are obloug, oval, concentrically 

 wrinkled, directed obliquely forwards and outwards, bordered 

 on the inner side by linear ridges, on the exteriör by a gently 

 curved slope, which gradually merges into the flattened margin 

 of the shell. The anterior scars are very small, nearly round. 

 Finally, there seems to be in the anterior part of the longi- 



