MUSEUM OF COMPARATIVE ZOOLOGY. 381 



disks, which only increase in thickness at a later time. The genus 

 Mycedium, which, even in its perfect condition, constitutes a thin, spread- 

 ing blade, may be compared, making allowance for the generic dif- 

 ferences, to a young spreading stock of Siderastrcea. In Mycedium 

 the mode of growth is very plain. A series of specimens collected by 

 M. Pourtales shows the beginning of such a coral community to be a 

 single individual, the margin of which gradually spreads ; from this 

 spreading edge are developed additional individuals in the trend 

 of the radiating partitions of the parent individual, spreading in 

 their turn, while they remain connected with one another and with the 

 central individual ; this process going on until the coral stock has 

 assumed its ordinary dimensions. Let us now conceive that the indi- 

 vidual Polpys, united as a coral-stock in Mycedium, should increase 

 vertically, as well as spread and multiply horizontally, the process of 

 elevation beginning in the centre, we should have a Siderastrcea. It is 

 worth noticing, further, that the original central individual, from which 

 the Mycedium community arises, is a diminutive Fungia, up to the time 

 when new individuals arise around its margin. I have before me such 

 young Mycediums, which might be mistaken for small specimens of 

 Fungiae, such as have been figured by Stuchbury and Milne Ed- 

 wards. "We are therefore justified in considering the genus Fungia as 

 an embryonic form of the type of Fungians, when we compare it to 

 Mycedium, Agaricia, or Siderastrcea ; and the propriety of assigning 

 to Fungia proper a lower position in a natural system than that be- 

 longing to the compound types of the family must be obvious to all. 

 The genus Zoopilus is only a Mycedium in which the individuals 

 of the community are more intimately blended together than in Halo- 

 mitra, thus forming a transition to Fungia proper. I have had an 

 opportunity of examining also the growth of Agaricia. "With the 

 exception of generic differences in its structure, it exhibits in its 

 growth the same features as Mycedium. The very youngest My- 

 cediums exhibit Turbinolian affinities, inasmuch as the interseptal 

 chambers are open from top to bottom and exhibit neither traverses 

 nor synapticules. 



Among Astrreans the early growth of a community takes place 

 in the same manner as among Fungians. Naturalists are accustomed 

 to consider the formation of the hemispheric masses of these corals as 

 arising from the formation of vertical buds around and between those 



