612 CAMBRIAN JELLY FISHES— WALCOTT. vol. xvm. 



canal in eacli lobe of the umbrella and in each interradial lobe, when 

 the latter is present. Oral plate quadripartite, with four oral arms 

 starting out from it, but whether these branch or not is unknown. A 

 second type of oral arms may be represented by the interradial lobes. 

 Type, BroolseUa alteniata. 



BROOKSELLA ALTERNATA, new species. 

 (Plate XXXI, tigs. 1-5.) 



The general form of the umbrella as now known varies from .^ub- 

 si^herical to a somewhat depressed convex disk. Following Hivckel, 

 the dorsal surface will be called the exnmhreUa ; the ventral surface, 

 the subumhrella ; the central section of the umbrella inclosing the 

 stomach and oral organs, the umhrella dhlt. 



In its original form the lobation of the exumbrella was more or less 

 clearly defined and varied. Individuals occur of nearly the same size, 

 with from six to twelve lobes; in some the lobation starts from the 

 center of the umbrella, w^iila in others a secondary system of lobes 

 appears from beneath the upper lobes and gives great irregularity to 

 the surface. The lobation of the umbrella is rarely, if ever, lost; it is 

 the dominant character in all specimens. 



The subumbrella varies to nearly as great a degree as the exum- 

 biella; strong ridges or ribs radiate from the center to each of the 

 principal lobes of the exumbrella. Sometimes the lobes separate above, 

 so that there is little more than the central umbrella disk with a series 

 of attached plates, like broad spokes in a w^heel. The least compressed 

 specimen is illustrated by Plate XXXI, figs. 2, 2a, and 2h. This, with Figs. 

 1, lo, 3, and 3a, may be considered the types of the genus and species. 



The gastrovascnlar system consists, as far as known, of a central 

 stomach and a radial canal, which passes from it to each exumbrella 

 lobe and interradial lobe. 



Xo traces of an oral opening have been seen. In a few specimens a 

 circular depression is seen at the base of the central axis, which was 

 probably the locality of the mouth at an early stage in the evolution 

 of the species and the development of the individual. One specimen 

 shows the presence of four oral arms, which form an oral plate where 

 they unite at the center. It is possible that the free interradial lobes 

 or arms, attached to the central axis beneath or between the umbrella 

 lobes, may have served the purpose of oral arms by carrying food to the 

 central stomach. This certainly appears to have been the case in Brooks- 

 ella con/usa, where there is no evidence of the presence of regular 

 oral arms. 



BROOKSELLA CONFUSA, new species. 

 (Plate XXXI, figs. 7, 7«, lb.) 



In the external form and appearance of the exumbrella this species 

 is similar to B. alteniata, but differs materially in the arrangement of 

 the lobes of the subumbrella. This is shown by the accompanying figure. 



! 



