8 FOSSIL MEDUSA. 



RELATION TO OTHER FOSSIL MEDUS/E. 



Since the publication of the notes on the Lower Cambrian medusa?, in 

 1891, 1 much more has been learned respecting the Middle Cambrian forms; 

 and it now appears to be possible to establish a family relation between the 

 Lower Cambrian Dactyloidites asteroides and the Middle Cambrian genera, 

 Brooksella and Laotira. The specimens of Dactyloidites are usually pressed 

 out flat in the lamina? of the slates, only a thin film remaining as a dark 

 stain on the slate. (See PI. XXVI.) In one instance some fine sand was 

 mixed with the argillaceous mud, and on a single large slab a few specimens 

 are a little convex. One of the best of these is shown by fig. 2 of PI. XXIV. 

 It is a beautiful specimen, in which the interior radial canals are indicated 

 within an inner portion of the lobes, which is separated from the dark sur- 

 rounding slate by a semitranslucent outer part of the lobes that strongly 

 resembles the sarcode of a recent medusa. Fig. 1 of PI. XXV is another 

 somewhat convex specimen, but it does not show any traces of the interior 

 structure. With the exception of fig. 2 of PI. XXIV, all the specimens of 

 Dactyloidites are very much compressed and distorted. For comparison 

 with Laotira cambria, compare fig. e of PI. XXVI with fig. 5 of PI. XXII; 

 fig. d of PI. XXVI with fig. 6 of PI. VI; fig. 1 of PI. XXV with fig. 10 of 

 PI. V; and fig. 2 of PI. XXIV with fig. 2 of PI. XXI. 



The foregoing comparisons, and many others that can be made with 

 the abundant material in the collections, establish the relation between the 

 Middle Cambrian genera and Dactyloidites. Of the two genera, Brooksella 

 appears to be nearer the latter than does Laotira. 



To the Lower Cambrian species of Sweden there is little apparent 

 similarity outside of their ordinal or subordinal relationship in belonging 

 to the Discomedusse. This applies to Medusina costata and M. princeps. 

 The position of M. radiata is much more doubtful. 



The well-identified Jurassic species are referred by Dr. Ammon to the 

 Discomedusae and the suborders Semostoma? and Rhizostomas. Except that 

 the Middle Cambrian species may possibly belong to the Rhizostomse, and 

 that three of the Jurassic are referred to the same, there appears to be no 

 opportunity for further detailed comparison. The transverse vertical section 



1 Fauna of the Lower Cambrian or Olenellus zone : Tenth Ann. Rept. U. S. Geol. Survey, pp. 587, 605, 606. 



