560 Karl A. Grönwall. 



N:o 4. Spiriferina Myiii n. sp. 



PI. XXVII, Figs. 3-7. 



Large shell, strongly arched, with its greatest width at the hinge- 

 line; its outline (determined along the hinge-line and the front line 

 of the shell) triangularly ovate. The cardinal area high and some- 

 what concave, of the same breadth as the hinge-line, bent backwards 

 so that it forms an angle of about 115° with the plane of articula- 

 tion for both valves, limited laterally by distinct, sharp margins. 

 The delthyrium occupies about one-quarter of the hinge-line, and 

 extends almost to the beak. 



The dorsal valve strongly arched, the greatest arching somewhat 

 below the beak which is small and faintly incurved and extends 

 somewhat beyond the hinge-line. The dorsal valve shows a strongly 

 vaulted median rib which, from the beak, rapidly expands, so that, 

 at the anterior margin, it occupies almost one-third of the same. 

 On each side of this rib there are 5 — 6 ribs, gradually decreasing 

 in size; the valve shows in the front a fairly evenly rounded out- 

 line. The ventral valve is strongly arched at the beak, but is flatter 

 anteriorly. The beak is small, but is strongly arched towards and 

 above the area. The sinus is broad and posteriorly fairly deep but, 

 anteriorly it becomes shallower, at the same time that it increases 

 in breadth, so that, at the anterior margin, it occupies nearly one- 

 third of the entire outline of the valve; here it is possible to observe 

 indications of two flat ribs, one on each side of the sinus. The valve 

 has 6 — 7 ribs on each side of the sinus, which decrease uniformly 

 in size. The front line of the ventral valve projects at the sinus 

 as an indication of a tongue, corresponding to the median rib of the 

 dorsal valve. 



The sculpture of the shell is well marked, with sharp, imbri- 

 cated laminæ of growth. In addition, the surface of the shell is 

 punctuated with fine pores, arranged in quincunx; the pores are 

 almost about as large as the intermediate spaces, and number about 

 5 — 6 per mm. 



Dimensions: Length of the hinge-line (width of shell) 25 mm. 

 Thickness of shell, 19 mm. Height of area (measured across the 

 umbo) 7 mm. Length of the ventral valve, 25 mm. Length of the 

 dorsal valve, 19 mm. 



Of this species we have, first, one complete specimen, freely 

 worked out from the rock, and the dimensions of which have been 

 given above, and which has been figured in PI. XXVII, Figs. 3 — 7, 

 and secondly, a fairly well preserved dorsal valve and a somewhat 

 corroded ventral valve, still remaining in the rock. They all 

 occur in the same piece of rock {N:o 209), in a white limestone, 



