96 



MEDUSA. I. 



southerly parts of its area of distribution. It is possible, accordingly, that the species just described 

 will prove in time to be only a northern giant variety of a species already known to science; but as 

 long as it cannot be referred with certainty to any known species, I prefer to describe it as an inde- 

 pendent species, for which I propose the name of PliiaUdiuvi tslatidiatm, because all the specimens in 

 hand have been found in the neighbourhood of Iceland. 



Chart XII. Finils of Phialiihum islandicum nov. sp. 



The species is quite distinct from PliinHdiiiiii Iioiiisplurricuiii^ not onl}- b}' its size and the 

 large number of tentacles, but also by tlie number of marginal vesicles never exceeding the number 

 of tentacle-s. Furthermore the mouth-lips are larger and more crenulated than in the case of Phialidiitjii 

 hemisph(e,ricum, and the gonads are longer. The sagittal sections through the tentacular bulbs show 

 (Plate V, figs. 2 and 3) that the ectoderm on the ada.xial .side of the bulb is more highly developed 

 in Phialidmnt hemisphmrictmt than in Ph. tslandicniit. The trace of an aba.xial jirocess on the base of 

 the tentacular bulbs in Plnalidinm islaiidiawi is not alwa>s distinct; in Ph. hcmispIurruKiii it is entirely 

 lacking. The shape of the marginal vesicles (see the sections, Plate IV, figs. 13 and 14) presents no 



