J887.] 395 [Fewkes. 



already been pointed out. It would be premature to suppose, how- 

 ever, that Velella has derived its peculiar anatomy from its descent 

 from a form like the parasitic Hydrichthys, rather than that Hy- 

 drichthys is a parasitic descendant of Velella ; while the acceptance 

 of the last mentioned theory would lead us to regard fixed hydroids 

 like Coryne as likewise descendants of parasitic forms with which 

 they have few resemblances. Indeed, we know next to nothing of 

 the egg and early growth of either Hydrichthys or Velella. We 

 have at all events found in Hydrichthys a near ally of Velella as 

 far as the hydroid is concerned, whatever may be the story told by 

 the early history of both. 



There is also another point long since known to those familiar 

 with the literature of the Hydromedusae, which is beautifully illus- 

 trated by Hydrichthys. Several naturalists have mentioned or 

 called attention to the resemblance of the medusae of hydroids of 

 very different form. We may have medusae so nearly related as to 

 be placed in the same genus, but their hydroids would otherwise 

 be placed in different genera. In Hydrichthys we have an illus- 

 tration of this principle. The medusa is similar to Sarsia, but 

 there is only a remote likeness between the attached hydroid Hy- 

 drichthys, and Coryne the hydroid of Sarsia. If a special student 

 of the hydroids was called upon to identify the parasitic hydroid, 

 he would consider its zoological distance from Coryne very consid- 

 erable, but a study of the medusa would lead him to a somewhat 

 different opinion of its zoological position. 



Do these facts of a difference in the form of the hydroids of 

 allied medusa-form gonophores, or vice versa, as sometimes hap- 

 pens, the diversity of medusae derived from similar hydroids, mean 

 anything morphologically? The question is an interesting one 

 and admits of several interpretations which, however, it is not my 

 purpose to consider this evening. There is one thing which has 

 a bearing on the subject, which I wish in closing to say in this 

 connection, viz. : the true affinities of the majority of genera of cam- 

 panularian or tubularian hydroids, or of Leptomedusce and An- 

 thomedusaz derived from the same, cannot be definitely made out 

 until both hydroid and medusa are studied together. 



Papers were also read by Dr. W. G. Farlow on the conception 

 of species in cryptogamic botany ; and by Prof. Wm. M. Davis 



on the physical history of the Somerville slates. 



