MEDUSA. I. 



of sad experiences, I take this opportunity to warn against the use of these substances for pre- 

 paration of medusae. 



In the case of such species, of which no modern descriptions have been given, I have carried 

 out thorough examinations of the morphology, as far as the state of preservation has rendered such 

 examination possible. Also in the case of well-known species I have in some cases discovered inter- 

 esting structures, hitherto unobserved. It is really astonishing, how most medusae are deficiently 

 known with regard to their morphology. It is a well-known fact, that the classification of the Lepto- 

 medusse is extremely difficult. I am quite aware, that a correct understanding of the systematical 

 problems must be searched for not only through hatching experiments, by means of which we get 

 knowledge of the connection between the hydroids and the medusae, but also through thorough com- 

 parative-morphological investigations. As a matter of course, I cannot, in the present paper, enter into 

 a discussion of the great and complicated systematical problems; my present knowledge of other forms 

 of medusae than the northern species is too deficient yet for such an undertaking. But it is my hope, 

 that my investigations, when in course of time they are followed by others, may contribute to that 

 understanding which, we may hope, must be reached once in the future. 



In some cases I have discussed the history of a species or genus in order to remove existing 

 confusion or to state the correct name of the species or genus in question. 



The number of species of Leptomedusae, identified with certainty, and occurring in the north- 

 atlantic area here dealt with, amounts to a little more than 25. In the material examined by me 15 

 species are represented, excluding the non-identified species of Obelia. They belong to the families 

 Laodiceidcz, Thaumantiadce, Mitrocomida: and Eucopida. The family of the sEquorida is not represented. 

 In order to give a picture, as complete as possible, of the north-atlantic fauna of medusae, I have 

 included the species of which no material has been at my disposal, but which, according to the litera- 

 ture, occur within the area in question. For such species, I give a short diagnosis, mainly derived 

 from A. G. Mayer, Medusae of the World, 1910, together with summary remarks on their distribution. 



The following 15 species have been examined: 



Chromatonema rubrurn Fewkes Halopsis ocellata A. Agassiz 



Laodicea undulata (Forbes & Goodsir) Tiaropsis multicirrata (M. Sars) 



Ptychogena lactea A. Agassiz Obelia nigra Browne 



Staurophora mertensii Brandt Phialidium hemisphaericum (Gronovius) 

 Melicertum octocostatum (M. Sars) islandicum nov. spec. 



Mitrocoma polydiademata (Romanes) Eutonina indicans (Romanes) 



Cosmetira pilosella Forbes Tima bairdii (Johnston) 

 megalota (Maas) 



Moreover the following species have been included (printed with Petit): 



Dipleurosoma typicum Boeck Eucheilota mactilata Hartlaub 



Agastra mira Hartlaub Saphenia gracilis (Forbes & Goodsir) 



Eucope globosa (Forbes) Kutima insignis (Keferstein) 



