TJie Relationship of the Tetracoralla to the Hexacoralla. 183 



A strict comparison of the life histories of these two species 

 cannot be made as yet, since Lindstrcemia whiteavesi is known 

 only from a single specimen (Foerste*i9o6: 312). On the other 

 hand, Brown (1909) made a detailed study of the development 

 of Streptelasma profimdum and found in siliceous specimens a 

 peculiar early growth of the skeleton which is different from that 

 of any other of the Tetracoralla yet investigated. He held that 

 the genus in its earliest growth was devoid of septa. In a speci- 

 men from I to 2 mm. in length, he found no septa, the individual 

 evidently having secreted a calycinal wall before any folds had 

 developed in the basal disk. These young coralla were merely 

 hollow cones with smooth inner walls. A little higher up in the 

 calyx four septa appear at once, but these are not plates dividing 

 the calyx into compartments, as is true of the Tetracoralla gen- 

 erally, but are low ridges in the cup. Succeeding ridges are 

 added in bilateral pairs, until a stage with twelve septa is attained 

 (Brown, 1909: 55, Fig. 2). After this stage of growth the septal 

 ridges become more prominent, are raised into lamellar ridges, 

 and finally, when eighteen septa are present, unite in the center 

 of the calyx. In no other of the Tetracoralla has a skeletal 

 growth been found without septa. This anomalous condition 

 could have been brought about through fossilization. The speci- 

 mens which Brown studied were all siliceous, i. e., pseudomorphs, 

 and the shortness of the septa may have been due to imperfect 

 silicification. It was shown, however, that the septa still pre- 

 served delicate, thin edges, a fact which may lead to the idea 

 that resorption of the skeleton took place during adult life. 

 This process, nevertheless, does not appear to have been one 

 operating upon the skeleton of Tetracoralla, for in all the other 

 genera examined the early septa are complete. 



Hypothetical genus Protostreptelasma. — Brown established an 

 hypothetical genus with the characters of an early stage in 

 Streptelasma profundum. He named this genus Protostreptel- 

 asma, "a. rugose coral having a hollow conical or horn-shaped 

 calyx, straight or slightly curved, without septa or having only 

 a few rudimentary ridges near the margin indicative of septa." 

 Such a genus was considered to be the ancestor of the Strep- 

 telasma line. 



The description of Protostreptelasma indicates a coralluni of 

 simpler structure than is known even in early youth among the 



