72 FOSSIL MEDUSAE. 



As in Hexarhizites, there are three zones which can be distinguished — an inner 

 or genital zone ; a median, smooth zone, and an external marginal zone. 



The mid-field shows a strong, bowl-shaped excavation, which plainly corresponds 

 to the gastral cavity. Its periphery is circular, but appears to be octagonal with 

 rounded corners, if one regards as its boundary points the points of contact of the 

 genital pouches and the stomach pouches. In the central part of the mid-field, where 

 it is most depressed, is the central mouth opening, surrounded by four oral arms 

 which are thrown outward to one side, over the edge of the umbrella. 



Arms appear as convex rolls, considerably elevated. They do not reach the 

 edge of the slab, but only about as far as the marginal tentacles. 



In the peripheral portion of the bowl-shaped mid-field arise eight radial ridges of 

 about 2 mm width, which gradually ascend from within outward until they reach the 

 steep peripheral descent of the genital zone, and here end in the form of eight convex 

 knots or papilla?,. These papillae, are the most elevated portion of the fossil, and 

 should therefore correspond to the thickest portion of the medusa disk. Each papilla 

 has about the shape of a three-sided pyramid with a blunt point. By a more exact 

 inspection of the mid-field and the adjacent genital zone, one is convinced that the 

 eight areas (of the shape of an isosceles triangle) which are dissected by the eight 

 radial ridges are alternately different. Four wide triangles alternate with four nar- 

 uower. The latter are only very slightly narrower, but in their peripheral portion 

 appear round, pit-like depressions, rather sharply defined, which are lacking in the 

 areas alternating with these, and which without doubt are to be interpreted as genital 

 cavities. The four narrower areas are interradial; the four broader ones alternating 

 with them are, on the contrary, perradial, and, because of their concave nature, are to 

 be regarded as gastral pouches. 



That the four broader areas which alternate with the genital fields are truly 

 correspondent to gastral pouches can not be doubted; while their peripheral rim is 

 sharply defined by a convex curved line, and the middle part appears strongly exca- 

 vated, their central end passes without interruption in the deep middle portion of the 

 mid-field, into the stomach cavity. 



In the peripheral portion of the smooth zone there are poorly defined wrinkles 

 which form fragments of concentric flutings, and are to be referred to the ring muscles 

 of the subumbrella. Besides this, almost no especial structure is recognizable on the 

 smooth zone. However, in certain lights, several very flue, straight, radial lines can 

 be seen which persist through the whole width of the smooth zone and indicate 

 radial canals. 



Since the families of this group are especially distinguished by means of the 

 different character of the radial canals, and since just upon this very detail of char- 

 acter no satisfactory degree of certainty can be attained, we will leave the question 

 of its family relationship open here. It almost seems, however, that it can not be 

 joined with any one of the known families. If this supposition proves correct, our 

 fossil group can be called Lithoseimna, and the related family Lithoseuneidai. 



