24 OLAF HOLTEDAHL. [SEC. ARCT. EXP. FRAM 



Lissairypa TWENHOFEIA 



Lissatrypa scheii nov. sp. 



PI. VII, figs. 9-12. 



This highly interesting fossil is no doubt the one most abundantly 

 occurring in the material. A great number of free specimens are pre- 

 sent, and some pieces of a brownish grey, very fine-grained limestone 

 filled with it, give us a good idea of the abundance of the specimen 

 in the rock. 



I posses specimens of very different size and the outer form changes 

 with the size. The adult specimen usually reaches a length of about 

 2 cm., a width of 1,8. The thickness varies much, in globose types it 

 amounts to 1516 mm., in flatter ones to 1213. The outline of the shell 

 seen from above also varies considerably; sometimes it is evenly rounded, 

 oval, but with the umbonal region more sharply lined and prominent. 

 Most specimens have a more polygonal outline, the margins on both 

 sides of the beak straight, meeting at the beak at very obtuse angles. 

 From these lines the lateral margins turn abruptly forwards, then gently 

 c'jrve towards the medium-line and meet at the anterior end in a nicely 

 rounded curve. The greatest width of the shell is generally at a point just 

 behind the centre. The convexity of the specimen varies, as mentioned above 

 greatly, the strongest convexity in the posterior part. Specimens with 

 relatively flat ventral and convex dorsal valve remind us as to the general 

 form, apart from the character of the surface, very much of small Atrypa 

 reticularis. As to the presence of a sinus and fold, there is consider- 

 able variation to be seen. In fact the specimens from the two local- 

 ities, where the specimens were found show a slight difference in this 

 respect. The specimens from Seal Bay show in the anterior third, 

 a more or less well marked sinus and fold. In some cases the 

 ventral valve in the front runs out into a tongue-shaped process. In 

 the more numerous specimens from the Reindeer Valley we very rarely 

 see a real depressed sinus at all. What often conveys the notion that a 

 sinus is indicated, is the fact that the margin in the front makes a 

 slightcurve upwards and forwards in every mature shell. 



1 The genus Lissatrypa (lissos = smooth) is proposed by Dr. W. H. TWENHOFEL 

 in the manuscript of a still un-printed work on the Ordovician and Silurian 

 Faunas of the Anticosti Island near the Mouth of the St. Lawrence River in 

 Eastern Canada. It is based on a small externally Athyris-\ike brachiopod, which 

 in its internal characters agrees with a true Atrypa. (Genotype Lissatrypa 

 atheroidea TWENH. nov. sp., from Zone 3 of the Jupiter River formation of the 

 Anticosti section, the equivalent of the Clinton). 



