718 MEDUSAE OF THE WORLD. 



star-like fossil, and sometimes two stars are joined by a single lobe. It appears to have been 

 gregarious, for Walcott records 42 specimens on a slab of slate 37 by 62 inches. There is of 

 course no proof that these fossils are those of medusae. 



Rhizostomites admirandus Haeckel. 



Rhizostomites admirandtts, HAECKEL, 1866, Neues Jahrbuch fiir Min. Geol. und Paleontol., p. 261, taf. 5; 1869, Zeit. fur wissen. 

 Zool., Bd. 19, p. 557; 1880, Syst. der Medusen, p. 647. BRANDT, 1871, Mem. Acad. Imp. Sci., St. Petersbourg, ser.y, 

 tome 16, p. I, planche I, figs. 1-4. VON AMMON, 1886, Abhandl. Math.-phys. Classe, Akad. Wissen Miinchen, Bd. 15, 

 p. 123, 158, 163, taf. 2, fig. 2. WALCOTT, 1898, Monographs U. S. Geol. Surv., vol. 30, p. 76, plates 40,42. MAAS, 1902, 

 Palaeontographica, Bd. 48, p. 306, taf. 23, fig. 2. VON AMMON, 1908, Geonostischen Jahresheftcn, Jahrg. 19, p. 174, fig. i . 



von Ammon, who has made the most thorough study of this fossil from the Jurassic 

 lithographic limestone ol Solenhofen and ot Eichstadt, Bavaria, has decided that it is 

 identical with Haeckel's Rhizostomites lithographicus, and that Leptobrac lutes trigonobrachius 

 is probably the same medusa turned over on its side. He also concludes that Haeckel's Hexa- 

 rhizites insignis is only a 6-rayed aberration of the same medusa. A thorough review of ihe 

 literature of this subject and excellent figures are presented by Walcott, 1898, loc. cit. 



According to von Ammon and Walcott the disk in Rhizostomites is round and as large 

 as 400 mm. in diameter, with 4 to 8 principal lobes and about 128 small marginal lappets of 

 various sizes, and indentations of the bell-rim marking the places of the 8 marginal sense- 

 organs. No marginal tentacles. A wide zone of circular muscles in the subumbrella, unbroken 

 in the rhopalar radii. 16 radial-canals, 8 rhopalar and 8 inter-rhopalar; and a circular canal in 

 the external third of the umbrella. A strongly marked, circular depression between the muscle- 

 zone and the arm-disk may indicate an inner ring-canal. 4 not very wide subgenital ostia, 

 with 4 opercula forming lappets. Probably 8 long, thin mouth-arms with crinkled appen- 

 dages, and apparently with a tassel-shaped tuft at the lower end. 



This is undoubtedly a Rhizostomous medusa which appears to belong to an extinct genus 

 related to the modern Rhizostomata triptera or lonfera. Maas, 1902, gives a remarkably clear 

 photograph of the margin showing one of the sense-organs, and he discusses the probable 

 form of the gonads. 



Brooksella alternata Walcott. 



Brooksella alternata, WALCOTT, 1986, Proc. U. S. Nat. Mus., vol. 18, p. 612, plate 31, figs. 1-5; 1898, Monographs V. S. Geol. 

 Surv., p. 23, plates 1-4. 



This fossil from the middle Cambrian shale from Coosa Valley, Alabama, is supposed to 

 be that of a medusa. They are 40 to 50 mm. in diameter. From 5 to 20, usually 5 to 8, more 

 or less distinct marginal lobes. No tentacles. A simple radial-canal in each lobe of the 

 umbrella. Oral plate quadripartite with 4 oral arms arising from it. Central stomach well 

 developed, but apparently there was no central mouth-opening. 



This form was possibly allied to the Rhizostomae and may have had habits similar to those 

 of Cassiopea. 



Brooksella confusa Walcott. 



Brooksella confusa, WALCOTT, 1896, Proc. U. S. Nat. Mus., vol. 18, p. 612, plate 31, figs. 7a-b; 1898, Monographs U. S. Geol. 

 Surv., vol. 30, p. 30, plate 3. 



In Brooksella confusa the marginal lobes do not unite at the center of the bell but join 

 irregularly, whereas in B. alternata they radiate from the center of the disk. This fossil is 

 found in the middle Cambrian shale of Coosa Valley, Alabama. 



Brooksella rhenana Kinkelin. 

 KINKELIN, 1903, Bericht Senckenberg Naturf. Gesell., Theil 2, p. 89, taf. I, fign. I, 2. 



An 8-lobed medusa from the middle Devonian of Ruplach. Only one specimen, showing 

 its exumbrella, was found, and this appears to me to be identical with, or at any rate very 

 closely related to, Walcott's Brooksella alternata. 



