APR 1^1893 



[Presented to the Oouuecticiit Academy of Arts and Sciences, May 20th, 1891.] 



XII. — The Development of a Paleozoic Poriferous Coral. 

 By Charles E. Beecher. (With Plates IX-XIII.) 



The origin and affinities of many groups of paleozoic corals are 

 still obscure. The main elements of the recognized system of 

 classification seem to be stable, yet so little is known of the 

 growth and structure of a number of important groups, that they 

 occupy a different place in almost every arrangement of the genera. 

 Each fact of development affords data which eliminate, to a degree, 

 the w^ant of knowledge concerning their origin and relations. Un- 

 less the growth of the organism is obscured by pronounced acceler- 

 ated or degradational features, its interpretation is simple, and 

 throws much light on its ancestral history. Paleozoic types in 

 general are least modified in their development by acceleration. 

 They usually show some marked expression of their jjrototype, and 

 also the succession of changes through which they have passed 

 during their evolution. 



The species here discussed was originally described as Miehelinia 

 lenticularis. Hall,* from the Lower Helderberg group of New York. 

 If Miehelinia is entitled to recognition, it will exclude this form, as 

 it is without tabular. PleurocUctyuni, as now defined, is more in har- 

 mony with these features, and, therefore, the species M. lenticularis 

 is here referred to this genus. The large calices and their constant 

 origin at the basal epitheca are not, however, essential characters of 

 Pleurodictyimi. The structure and growth of this species indicate 

 that it represents one of the simpler types of poriferous corals. For 

 this reason, its development is without the numerous modifications 

 necessary in more complex forms, and its laws of growth are not 

 complicated. 



Development of Plenrodictyum lenticulare. — The nepionic stage is 

 well marked. It comprises the growth of the corallum to the com- 

 pletion of a simple initial cell. This primitive cell or nepionic stage 

 (Plate IX, figure 1, V, 8) has the foi'm of an oblique inverted cone 

 flattened on one side. The flattened area represents the lower or 

 attached side, and the oblique base of the cone is occupied by the 



* Twenty-sixth Kept. N. Y. State Museum of Nat. Hist., p. 113, 1874. 

 Tr.-vns. Conn. Acad., Vol. VTII. 28 July, 1891. 



