BIIIAXG TILL K. SV. VET.-AKAU. HANDL. BAND. 3. N:0 12. 19 



Obolella sagittalis (Salt.) Dav. 



Pl. III, figs. 36—41. 



Obolella sagittalis Dav., Geol. Mag., Vol. V, 1868, p. 309, pl. 

 XV, figs. 17—24. 

 )) » Dav. Monograpli of Britisli Silurian Brachio- 



poda, p. 339, pl. L, figs. 1 — 14. 



Xearly circiilar, a little wider than long, ratlier broader an- 

 teriorly. Surface siiiooth, polislied. Yentral A^alve moderately 

 and uniformly convex; beak situated near the posterior margin, 

 bent baclvAvards and perforated by a minute, round foramen, 

 which, however, I have seen only in one single specimen. Be- 

 tween the beak and the margin there is no distinet area to be seen 

 in this specimen, which is very well preservcd, and therefore it 

 probably never occurs. I have not found any interiör of the 

 ventral valve, but internal casts are common. They show a 

 rounded groove at the apex, and two smaller ones behind that. 

 In the interiör of the dorsal valve two projecting musciilar scars, 

 of variable size and shape, are situated close to the posterior 

 margin. Between them a nearly linear, but slightly widening 

 mesial ridge extends from the margin to about the middle of 

 the valve. Its length varies, so that in some specimens it does 

 not reach the middle, while in others it exceeds it. Close to 

 the mesial ridge there is, on either side, a smaller, almost pa- 

 rallel ridge. The anterior muscular scars I have not been able 

 to discern. — The dimensions of the shell are always small, the 

 larger diameter hardly exceeding 4 mm. Some specimens measure: 

 Length 3 mm. Width 4 mm. 

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Though the Scandinavian species above describcd does not 

 in every respect agree wdth the admirable figures and descrip- 

 tions given by Mr. Davidson, the difierences do not seem to be 

 greater than to admit of an Identification with the English species. 

 Mr. Davidson tells us that he has not seen in the English spe- 

 cies any groove for the passage of a pedicle, but I do not think 

 it impossible that more ample materials will show this character 

 to be common to both, and that this disagreement will thus be 

 removed. The interiör of the ventral valve agrees completely, 

 judging from the numerous internal casts I have had an oppor- 



