308 PALEONTOLOGY 



ment in the same cycles as the septa, except that the first 

 and second cycles are not distinguishable. 



Dr. W. D. Lang, of the British Museum, has shown 

 that these costae pass through a series of stages in on- 

 togeny. On the peduncle the costae are low and faint 

 (they are also only twelve in number, the last cycle of 

 septa not being yet developed); as growth proceeds they 

 become stronger, but remain quite smooth. This 

 anagenesis of costae continues for a distance, and is fol- 

 lowed by a sudden catagenesis, the costae almost disap- 

 pearing. A second long period of anagenesis follows, but 

 this time the costae are not smooth, but have a roughened 

 or etched appearance. This is as far as the development 

 goes in P. centralis, though if it continues to grow in 

 cylindrical form there will be several temporary stoppages 

 of growth, followed by a rejuvenation a fresh start being 

 made from a lower stage of anagenesis than had been 

 reached before the stoppage. 



In other species of Parasmilia, the anagenesis of etched 

 costae is followed by another sudden catagenesis, and 

 that by a third period of anagenesis in which the costae 

 have a granulate appearance. There are thus three 

 types of costae, and if we recognize three stages in each 

 (low, medium, and high), altogether nine stages. No 

 species shows all nine. Those which come earliest in 

 time show the first four (P. serpentina] or six (P. centralis) ; 

 later species skip the first three stages (tachygenesis), 

 and show the next five (P. grannlata) or six (P. gravest); 

 still later forms (P. cylindrica, P. mantelli) start straight 

 away with granulate costae, and show the last three 



