26 FOSSIL MEDUSA. 



compared with such regular forms as are represented by figs. 1 and 3 of 

 PI. I. The larger proportion of the specimens possessing the latter char- 

 acters are flattened on the lower side so as to form a compressed disk, but 

 a few specimens retain some of the original fullness and have a transverse 

 spheroidal outline. 



umbrella corona. — The line of the corona furrow is suggested by the ring 

 about the central disk (PI. I, figs. 1, 2, and 3). It is quite probable that 

 this is correct, as the sutures between the exumbrella lobes cut back to the 

 disk in nearly all, if not all, the specimens (PI. I, figs 1, 2, 3; PI. Ill, figs. 

 1, la, 5, 6). The ring is not always present, but it usually is in the more 

 perfect specimens, where the distinct exumbrella lobation does not continue 

 to the center of the disk. 



The umbrella margin is deeply lobed, following the rim of the exum- 

 brella lobes. In such specimens as are represented by fig. 5 of PI. II and 

 figs. 3, 6, etc., of PI. Ill, it could have been little more than the irregular 

 rim of the various exumbrella and mterumbrella lobes. The presence of 

 organs of sense on the margin is not known from the fossil specimens. It 

 is only by the relations of the species to the recent Discomedusse that we 

 may assume that they were present. Haeckel says : 1 



The umbrella margin is the most important part of the neurodermal system in 

 all medusa?, both morphologically and physiologically, as in it the most important 

 animal organs — organs of sense, nerves, and muscles — attain their highest develop- 

 ment. The central part of the nervous system and the tentacles especially are always 

 originally situated in the umbrella margin. The umbrella margin is also of great 

 importance for classification, as it is chiefly on it that the variations of formation 

 appear which lead to the distinction of genera and species. In fact, the distinction 

 and nomenclature of the two principal divisions of the class Medusre, of the two sec- 

 tions Craspedota and Acraspeda, are taken from the umbrella margin, which presents 

 important and striking diversities in the two sections. The "velum" is characteristic 

 of the former, the "lobe corona" of the latter. 



In Brooksella alternata a,nd associated medusae no tentacles have been 

 observed. Organs of sense, touch, smell, vision, and hearing probably 

 existed. We can imagine the existence of sense clubs from analogy with 

 recent Discomedusse, but they are not preserved in the fossil state. All 

 such organs appear to have been destroyed in the process of fossilization. 



Gastrovascuiar system. — In all medusa- the gastrovascular system or intestinal system 

 is divided first of all into two principal sections, a central and a peripheral part. For 



1 Report on the deep-sea medus;i> : Voyage of H. M. S. Challenger; Zoology, Vol. IV, 1882, p. xlii. 



