XIII. — Symmetrical Cell Development in the Favositid^. 

 By Charles E. Bebchbe. (With Plates XIV, XV.) 



The majority of compound corals included in the I^avositidce are 

 composed of polygonal, prismatic cells or corallites in juxta2)osition. 

 When, however, these cells become free, their form is cylindrical. 

 The polygonal form of closely arranged cells is therefore explained 

 as the natural result of crowding. 



The species Pleurodictyimi lenticulare, Hall, sp., is an example of 

 simple cell growth and multiplication. In the development of this 

 species, as shown by the writer in the previous paper, the initial 

 corallite is first conical. The growth of a peripheral series of buds 

 results in changing the sub-circular section of the parent corallite 

 into a polygon. The buds are angular on the sides in juxtaposition 

 to the parent cell and adjacent buds, but on the free portion of their 

 periphery they are cylindrical. The subsequent growth of peri- 

 pheral buds brings the first series wholly within the cbrallum, and 

 they are then polygonal in section like the parent corallite. 



In compact corals with long cell tubes, as Michelinia and Favosites, 

 there is a maximum limit to the size of the corallites. Thus, the 

 form of the cells which have reached this limit of diametral exten- 

 sion is that of equal hexagonal prisms. This is of course due to the 

 well known fact of six equal tangent circles about a central circle of 

 the same size. Then from crowding, or from the elimination of the 

 interstitial spaces, they assume a regular hexagonal form. The 

 specimen of Clelstopora geometrica, illustrated by Edwards and 

 Haime,* represents the maximum size of the cells and their equal 

 development in this species. Although the tubes are not long, the 

 calices are nearly of the same size, and regularly hexagonal. 



After the completion of a circle of calices about the parent cell of 

 the corallum, enlargement takes place, (1) by buds from the peri- 

 phery, and (2) by intermural gemination. The first is not attended 

 by any phenomena differing from the production of the primary 

 circlet of calices about the initial cell. The second takes place 

 under other conditions, and is the chief method of increase in the 

 growth of large corallums having numerous corallites. 



* Monographie des Polypiers Fossiles des Terraiaes Palceozmques, p. 252, pi. 17, fig. 

 3. 1851. 



Trans. Conn. Acad., Vol. VIII. 29 July, 1891. 



