8 NEW YORK STATE MUSEUM 



Favosites turbinatus Billings 

 Plate I, figures 1-7 



The purpose of this note is to describe an exceptionally perfect 

 specimen of this coral from the Onondaga limestone of western 

 New York. The specimen, which is a young colony, is remarkable 

 for several features, the most important of which is the covering 

 of most of the corallum with opercula. The opercula are sub- 

 circular plates which overlap each other like the tiles of a roof 

 from above downward. Their surface is flat or low convex, mostly 

 smooth or showing an obscure concentric structure due to the build- 

 ing up of the opercula of gradually widening layers. The center 

 shows frequently a low knob in mature opercula, while the younger 

 ones possess in its place a depression, or in the smallest ones even 

 a perforation. It appears from this structure that, as Rominger 

 has stated, 1 the young opercula were provided with a central open- 

 ing which finally was closed by a solid nodular piece. Some 

 opercula show the structure reproduced in figure 6. Here a dis- 

 tinctly concentric annular structure with a central depression is 

 revealed after the last, outer smooth layer has been scaled off. 

 This indicates that the last stage was not only the filling of the 

 depression but also the covering of the operculum with a final 

 smooth layer. Toward the upper younger portion of the corallum 

 the opercula become smaller and on the top they apparently were, 

 for the most part, absent. They are therefore a more or less 

 gerontic feature of the corallites, and at the lower, older portion 

 they overlap so tightly that the corallites may there have been 

 closed definitely. At the base they have coalesced into a solid mass 

 so that the different opercula can hardly be distinguished. They 

 thus effectually sealed that part of the corallum which gradually 

 sank into the mud as the growing corallum grew heavier. 



Where the operculum is broken out, a low conical calyxlike 

 depression is seen, which contains from 12 to 16 radial ridges, 

 resembling septa. They are most distinct in the smaller calyces 

 where they reach to the bottom, while in the older corallites they 

 are mostly reduced to the knoblike outer terminations which again 

 in many corallites are continued outward beyond the crest of the 

 calyx into rapidly tapering low ridges sometimes even upon the 

 next operculum below. The undersides of the opercula are fur- 

 nished with short peripheral ridges and grooves which fit into the 



1 C Rominger. Geological Survey of Michigan, v. 3, pt I. Geology, pt 2. 

 Paleontology — Corals, p. 26. 1876. 



