318 bulletin: museum of comparative zoology. 



7. Heliolites interstinctus. 



8. Paleofavosites aspera. 



9. Atrypa imbricata var. 



10. Coelospira duboysi (Verneuil). 



11. Dinobolus davidsoni wSalter. 



12. Orthis bouchardi Verneuil. 



13. davidsoni Verneuil. 



14. Rhynchonella aprinis Verneuil. 



15. Schuchertella pecten (Linne). 



16. Encrinurus punetatus Wahlenberg. 



17. Proetus cf. distans. Lindstrom. 



18. planedorsatus Schmidt. 



19. Leperditia schmidti Kolm. 



There have also been recognized Clathrodictyon vesiculosum, Favosites 

 forbesi, Halysites catenularia, and Pentamerus borealis. 



(b) Borealis banks. This is one of the most remarkable deposits 

 of fossil shells I have ever seen. They are almost wholly Pentamerus 

 borealis, generally single valves, although in the western outcrops it is 

 possible to obtain entire shells. 



Respecting the continuity of this zone it is not possible to speak 

 with certainty; but the general facts of the distribution of marine 

 organisms would lead to the conclusion that it is not of wide extent 

 and it seems very probable that it thins westward. It also appears 

 quite probable that in places the character of deposition which gave 

 rise to the Jorden beds may have persisted upward while the shell- 

 banks grew around, or within such places, that is, the banks are of a 

 more or less local distribution. 



The zone makes its first appearance west of Lake Peipus near St. 

 Simon's church, and, narrowing toward the east, the belt of its out- 

 crop reaches the coast a few miles south of the city of Hapsal, and finds 

 its further continuation on the island of Dago. Schmidt, {Loc. cit., 

 1882, p. 526) gives the thickness as forty feet. 



Where the division is typically shown little else is to be seen other 

 than myriads of Pentamerus borealis and a few corals. The number 

 of the former is simply inconceivable. The rock is wholly a mass of 

 the shells, which are chiefly preserved as casts. Where the shells are 

 not the dominant component, the rock is either a limestone or a dolo- 

 mite. 



About the 'best locality to see this division, and the one which has 

 been selected as the type-locality of the Tamsal formation, is north- 



