580 Karl A. Grönwall. 



Productus longispinus Sow. is a fairly variable form, of which 

 a number of different species have been drawn up, but which Da- 

 vidson, however, groups together. Its stratigraphical distribution 

 is, too, fairly wide, so that it can hardly make a very valuable 

 contribution to the determination of the horizon. Productus lobatus 

 Sow. which TscHERNYSCHEw (1902, p. 263) mentions from the Ompha- 

 lotrochus and Schwagerina horizons of Russia, is considered by 

 Davidson to be a variety of Pr. longispinus Sow. 



N:o 19. Productus compressus Waag. 



PI. XXVIII, Figs. 15—18. 



Waagen, 1882—84, p. 710, pi. 81. Figs. 1—2. 



A specimen of a form appertaining to the Pr. striatus group, 

 and found in the very fossiliferous limestone, N:o 15A from the 

 Mallemukfjæld, ought probably, although somewhat doubtfully, be 

 referred to this species. 



The forms belonging to the striatus group are fairly closely 

 related to each olher, and here, as in the case of the Cora group. 

 Gröber denies that we are entitled to establish more than one 

 species. 



The specimen we have figured here is triangular in outline, 

 and wants a proper hinge-line. Of the ears there remain only 

 faintly discernible remains, which show that they have a direction 

 vertical to the valve. The beak is pointed, and the umbonal portion 

 strongly compressed. 



The ventral valve has no indication of a sinus, and, from the 

 beak it is arched in an even curvature, until it meets the concave 

 dorsal valve where it bends sharply anteriorly. In consequence of 

 this form of the shell, the visceral room becomes low and small. 



The sculpture of the ventral valve is not preserved, as the 

 valve, as is usually the case with this group, is very thin, but the 

 mould clearly shows irregular wrinkles of the same form and appea- 

 rance as those shown in Waagen's figure, although they are not so 

 numerous. The mould also shows traces of about 4 spines on the 

 one flank. 



The dorsal valve is ornamented with fine slriæ which increase 

 in number through intercalation, and spread fan-wise over the entire 

 surface of the valve, so that they meet the margin at about right 

 angles. 



The characteristics in which our specimen differs from Waa- 

 gen's are: the size — the length, in specimens from the Productus 

 limestone of the Salt Range reaching as much as 55 mm. — and 

 also the arching of the ventral valve. This latter character is slated 



