MIDDLE CAMBRIAN. 33 



points: In the simple forms (PL V, figs. 1 and 11; PI. VI, figs. 4a and 7a, 

 etc.), the subumbrella lobes radiate from the central axis to the exnmbrella 

 lobes, as in Brooksella alternata; but with the introduction of a tendency to 

 irregularity in the arrangement of the subumbrella lobes at the center, all 

 order and system is soon lost. The exumbrella lobes of fig. 3, PI. IX, are 

 systematically arranged about the center; but the subumbrella lobes show 

 a slightly transverse arrangement, so that they do not meet at a common 

 center; this is carried further in figs. 5, 6, and 7 of PL IX, and 5 of PL X, 

 until a distinctly bilateral arrangement is developed. In many individuals 

 there is no regularity (PL XI, figs, la and 2a, etc.), and in the extreme forms 

 (PI XIII, figs. 5 and 6), there is an irregular network of subumbrella 

 lobes and oral arms. 



umbrella corona. — The corona fuiTOw has not been recognized in any indi- 

 vidual that could with certainty be referred to this species. The umbrella 

 margin of the simple forms is not sufficiently well preserved to show any 

 organs of sense, and there is no defined margin in the complex forms. No 

 tentacles have been observed. 



Gastrovascuiar system. — In the simple forms the radiating canals of the exum- 

 brella lobes appear as in Brooksella alternata, and they unite at the center of 

 the central axis (PL XVII, fig. 6 ; PL XXI, fig. 2 ; canals and central cham- 

 ber). The termination of the radial canals is shown in figs. 2 and 6 of PL V, 

 1 and 3 of PL VI, and 3a, 3c, and 4a of PL XVIII, and their course in the 

 exumbrella lobes by fig. 7 of PL VIII and fig. 2 of PL XXI and in the 

 transverse sections on PL XXIII, figs. 1, 3. In the complex forms the 

 irregularity of the exumbrella lobes influences the interior canals (PL XVII, 

 fig. 3a), and even, in some cases, where the exterior is nearly symmetrical 

 the interior shows irregularity in the arrangement of the canals (PL XXIII, 

 fig. 7). Often the canals are not so well preserved in the more complex 

 forms, but a careful examination usually shows traces of them, and in worn 

 specimens they are sometimes clearly shown. The only specimen preserving 

 the radial exumbrella canals, flattened in the shale, is but a film in which 

 the canals are traced by dark, iron-stained bands about l mm wide (PL VIII, 

 tig. 7). The medusa is completely pressed out in an oval form 4 cm by 7 cm 

 in diameter. The 30 to 34 radial canals terminate at a central area, H mm 

 by I7 mm in diameter, that corresponds to the central stomach, as shown by 

 figs. 6 of PL XVII, 2 of PL XXI, and 1 and 3 of PL XXIII. This central 



MON XXX 3 



