Eoyal Society. 531 



which is towards the stem, the other below from the front part of 

 the base of the cell. They were conical in one species, club-shaped 

 and articulated in the other, and consisted of an external horny 

 membrane open at the apex, and an internal delicate membrane in- 

 closing a cavity, all these being continuous with the corresponding 

 parts of the stem. At the apex of each, and capable of being pressed 

 through the aperture, lay a number of thread-cells. The second 

 kind of organ was present in the species with conical processes. It 

 consisted of a stem proceeding from the pedicle of the ovary, bearing 

 a series of conical bodies, having the same constitution as those just 

 described ; the whole bearing a close resemblance to the prehensile 

 organs of the Diphydae. 



The following table exhibits the homologies of the several families, 

 which must be regarded as by no means so distinct as hitherto sup- 

 posed, but rather as members of one great group, organized upon 

 one simple and uniform plan, and even in their most complex and 

 aberrant forms reducible to the same type. 



Stomachs identical in Structure throughout. 

 Medusae. Physophoridcs. Diphydae. Sertularidm. Hydra. 



Disc Natatorial organ .... Natatorial organ. 



_ , f Canals of natatorial ) Canals of natatorial 



Canal* 1 organ / organ. 



Common cavity . . ") , oo/.n„i„u a,,A „nm i 



Canals of branches Common tube . . . . { ^^ tube ) ^'^'^'^^'^ "^ **^^'"- 



(Hhiz.) J 



Bract Polype-cell. 



.^.>~ 



Tentacles,! {''^^oS .^''^."5} Qval tentacles. 



2 Prehensile organs Clavate organs .... Tentacles (?). 



C Generative sac Generative organ.. . Generative organ< 



Generative organs < Natatorial organ of I j Natatorial organs 



L generative sac . . / ' " " I (Coryne). 



Marginal vescicle ? ? ? 



"On the Microscopic Structure of the Scales and Dermal 

 Teeth of some Ganoid and Placoid Fish." By W. C. Williamson, 

 Esq. Communicated by Edwin Lankester, M.D., F.R.S. 



The author commences his paper by stating that the structure 

 and modes of growth of fish-scales have been studied by many ob- 

 servers, especially by Leeuwehhoek, Agassiz, Mandl and Owen. 

 The first of these considered each scale to consist of numerous 

 superimposed laminae added successively to the inferior surface. 

 This view has been revived, with some important modifications, by 

 M. Agassiz, and especially applied to the scales of ganoid fish; 

 which he showed to consist of laminae of true bone, usually 

 covered with enamel (email), the latter often resembling the den- 

 tine of fishes' teeth. M. Mandl denied that ganoid scales had 

 been formed by such successive additions of laminae ; and Professor 

 Owen also opposed the idea, that they had merely been the result of 

 successively excreted deposition. The author then proceeds to the 

 examination of the scales of the following genera and species : — 

 Lepidosteus osseus, Lepidotus semiserratus, L. Mant^lli, and L.fitn," 

 briatus, Seminotus rhombifer, Pholidotus Leachii, Ptycholepis BoU 



2 M 2 



