196 ROBERT PAYNE BIGELOW ON 



Fewkes it is impossible to identify his species positively. But a comparison of his 

 figures with living specimens of both sexes of P.frondosa shows at once that the two 

 species are distinct ; while a comparison with C. xamachana shows so close a resemblance 

 that I am inclined to think that Fewkes has discovered one of the varieties of our species, 

 described in the next section. Not only is P. frondosa perfectly distinct from C. xama- 

 chana, but I think we are justified in retaining the former, for the present at least, in a 

 separate genus ; and there can be little doubt that Lamarck's Cassiopea frondosa dwell- 

 ing in the "Ocean of the Antilles" with its " margine decem-lobata" is none other than 

 Agassiz's Polyclonia frondosa. Therefore, even if it should be proved that the form 

 described by Fewkes is the same as the subject of the present memoir, the name that I 

 have given to it will hold, nevertheless, as the designation of the species. 



Variations. If we compare the average dimensions of various organs, expressed in 

 thousandths of the diameter, with the maxima and minima, as may be done by examining 

 the third, fourth, and fifth columns in Table 1, p. 201, it becomes evident that there is a 

 very considerable amount of variation in the relative size of parts of C. xamachana. 



In the oral arms, not only does the relative size vary, but the number and the 

 arrangement of the branches are both variable. Moreover this variability exists between 

 the different individuals. In nineteen specimens examined the maximum number of 

 branches found on one arm was sixteen, the minimum nine, and the greatest difference 

 on any one individual was four. 



The most striking variations in C. xamachana, however, are to be found in the 

 structures at the margin of the umbrella. These are highly variable in this species, and 

 have been found to be variable, although to a less extent, in other medusae. It is 

 unfortunate, therefore, that in his beautiful systematic work on the medusae Haeckel 

 should have found himself forced to distinguish the genera chiefly by differences in the 

 marginal structures. He himself notes the variability in the number of parameres of 

 Polyclonia frondosa. Agassiz and Mayer ('99) found in one specimen of C. ndrosia 

 eighteen rhopalia, and in another twenty-two. 



The number of rhopalia was counted in twenty-seven specimens of C. xamachana. Of 

 these ten were found to have sixteen, the typical number for the genus, and twelve had 

 more than sixteen, three having seventeen, and three more, eighteen. The largest 

 number on one individual was twenty-three. There were five specimens with fewer than 

 sixteen rhopalia, but only two had less than fifteen, and both of these showed correlated 

 abnormalities in the mouth parts and subgenital cavities. One had fourteen rhopalia, four 

 oral arms, and two subgenital cavities and gonads. The other had ten rhopalia, only five 

 oral arms, with three oesophageal canals leading from the stomach to the canal system of 

 the arms, and three normal subgenital cavities and one very small vestigial one (Fig. A.) . 



