It is a most agreeable duty for me gratefuUy to acknowledge tlie friendly readiness with which Profes- 

 sor Tameelan Thorell oomplied with my request to deseribe this remarkable Silurian fossil. To this desoription I 

 have contributed very sliglitly. The third, foiirth and fifth sections of this paper are entirely by Prof. Thorell, 

 with whose opinions I oompletely agree. My oordial thanks are also due to Dr G. J. Hinde who kindly under- 

 took to revise the English of the proof sheets. With his usiial skill Herr G. Liljeval! has exeonted the plate 

 with all its details on stone. 



G. Lindström. 



