PERIPHYLLA. 25 



Both Vanhoffen (: 02 a ) and Maas (: 03, : 04°) have agreed that the two 

 forms are characterised as follows: — in P. dodecabostrycha the bell is low and 

 only the central portion of the gastrovascular system is pigmented, so that the 

 gonads are visible from without. In P. hyatinthina the bell is high and the 

 distal portions of the canals are so heavily pigmented as to conceal the gonads. 

 As to the first of these distinctions, the evidence seems to be by no means 

 convincing, although there is no doubt that specimens described as P. hyacin- 

 thina do average rather higher than P. dodecabostrycha. Vanhoffen (: 02 a , p. 23) 

 has given the following proportions of height to diameter for the two species : — 

 for P. hyatinthina 44 : 23, 39 : 26, 35 : 21, for P. dodecabostrycha 27 : 18, 12 : 10, 

 20 : 13. These figures as they stand are misleading, since they suggest too 

 great a diversity in the case of the larger (P. hyatinthina) measurements. By 

 reducing them to a common standard, i. e. in terms of the diameters, we can 

 form a better judgment of their true meaning. We then find the proportions 

 of P. hyatinthina to be 1.9 : 1, 1.7 : 1, 1.5 : 1, and of P. dodecabostrycha 1.55 : 1, 

 1.5 : 1, 1.2 : 1. It is clear, then, from these figures, to which, as I have found, 

 more might be added without changing their import, that in this character 

 there is no discontinuity between the two; indeed, one specimen of P. 

 dodecabostrycha is rather higher in proportion to its diameter than one of P. 

 hyatinthina. This character seems to be subject to considerable individual 

 variation. Certainly when it is analyzed it forms no basis for any safe specific 

 distinction, and all we can safely say is that large individuals are as a rule 

 rather higher than small ones. The form of the bell is probably no more im- 

 portant in this connection, for Maas (: 04 c , p. 47) has already recorded a typical 

 specimen of P. hyatinthina of which he says, " ombrelle dans sa mesoglee pas 

 ci pointue que cela est dessine generalement pour hyatinthina." 



The extent of the endodermic pigmentation does not seem much more 

 valuable as a specific criterion, for not only is this character variable among 

 Medusae in general, but also there is evidence that in Periphylla the 

 amount of pigmentation increases with growth. Thus while in two very 

 small specimens in the present collection the pigment is entirely restricted to 

 the central stomach, not even the ring sinus .showing any trace of color, in 

 five somewhat larger specimens from the Gulf Stream, and four, nil about 20 

 mm. in height, from off the Alaskan coast, as well as in most described 

 specimens of P. dodecabostrycha, the ring sinus is somewhat pigmented, though 

 not enough to hide the gonads; and in all large specimens of the genus \ el 

 recorded, at least by recent authors (except those referable to P. regina) the 



